The author investigated 125 lower limbs of 69 Japanese cadavers from his dissecting room, and obtained the following results. 1. Definition of the perforating arteries and the medial perforating branches. As generally accepted, only those arteries, that perforate the adductors close to the linea aspera, are called perforating arteries. Such arteries, as those piercing the adductor maguns far from the femur, are provisionally called "medial perforating branches". The first perforating artery (Ⅰ) appears backwards from between the adductor minimus and the main part of the adductor magnus. The second perforating artery (Ⅱ) perforates the main part of the adductor magnus above or at the proximal third of the origin of the short head of the biceps. The third perforating arteries (Ⅲ) perforate the adductor magnus at the height of the middle and the distal thirds of the biceps origin. In some cases, a twig whose course and distribution are the same as those of the third perforating arteries arises from the distal part of the femoral artery or from the popliteal artery. Such are also included in the third perforating arteries. "Superior medial perforating branch" arises from the first perforating artery, the profunda femoris artery or the medial circumflex artery, pierces the adductor magnus and appears at the upper medial part of its posterior surface. Generally it occurs as one twig. "Inferior medial perforating branches" arise from the muscular branches to the adductor magnus and appears backwards at the lower part of the muscle, as one to three twigs. (Table 9). 2. The total number of perforating arteries (Tables 1, 2 and 3). They arise from the profunda femoris as 2-6 branches, in most of the cases as 3-5 (average 4.02). As they sometimes divide into 2 or 3 branches before they pass through the adductors, the number of branches, piercing the adductor magnus, is more than the former: 2-7, in most of the cases 4 or 5 (average 4.66). Furthermore, if the perforating arteries of the femoral and popliteal origins are included, the average number of the arteries, passing through the adductor magnus, is 4.85. 3. The number of each perforating artery (Tables 5 and 6). I is generally single, rather rarely two (30 limbs, 24.0%) or three (3 limbs, 2.4%) in number. The supernumerary first perforating arteries are its ascending branches, that pierce the muscle separately. Ⅱ also is generally single, rather rarely two or three (17.6%). Whereas, Ⅲ is generally two or three in number. In one case Ⅲ was missing. 4. The origin and branching of the perforating arteries are presented in Fig. 2. In the majority of the cases, the first and the second branches become Ⅰ and Ⅱ respectively, and Ⅲ are derived from the third branch and on (Type Ⅰ, 72.0%). Besides, the type Ⅱ (15.2%), in which the first branch divides into Ⅰ and Ⅱ, and type Ⅲ (12.0%), in which Ⅱ are derived from the first and second branches, are not rarely found. 5. Generally, Ⅰ and Ⅱ pierce the adductor brevis and the adductor magnus, while Ⅲ the latter muscle only. (Table 4). 6. Distribution of the perforating arteries and the "medial perforating branches". (Table 7 and Fig. 4). Generally the ascending branch of Ⅰ is distributed to the lower part of the gluteus maximus and its neighbourhood, the descending branch (the main branch), as the principal source of the arterial supply of the hamstring muscles, supply the long head of the biceps, the semitendinosus, the semimembranosus and the sciatic nerve, and Ⅱ and Ⅲ supply the short head of the biceps and the vastus lateralis and intermedius. The "superior medial perforating branch" enters the proximal part of the semimembranosus, and the "inferior medial perforating branches" supply chiefly the belly of the semimembranosus. 7. Besides the perforating arteries and the medial perforating branches, the hamstring muscles are supplied by some twigs : the transverse branch of the medial circumflex artery supplies the proximal part of the semitendinosus and the long head, and some twigs from the distal part of the femoral and from the popliteal arteries supply the distal part of the hamstring muscles. 8. Arterial pattern of the back of the thigh (Table 12, Figs 4, 5, 6) The lower part of the gluteus maximus receives branches from Ⅰ (100%), the vastus lateralis and intermedius from Ⅱ (95.2%) and Ⅲ (85.6%), the short head of the biceps from Ⅱ (57.6 %) and Ⅲ (99.2%), and the sciatic nerve from Ⅰ (82.4%), each with little variations. On the other hand, the arteries to the belly of the long head the semitendinosus and the semimembranosus are fairly variable. Normally, their main source is the descending branch of Ⅰ, and the inferior medial perforating branches (M) and sometimes Ⅱ, rarely Ⅲ as well, supply only a small part. But not rarely the descending branch is small or missing and substituted by M or Ⅱ. In this respect various arterial patterns of the back of the thigh are classified into five types: two types with developed Ⅰ, normal type (74 limbs, 59.2%) and paranormal type (19 limbs, 15.2%), type with developed M (14 limbs, 11.2%), type with developed Ⅱ (8 limbs, 6.4%) and mixed type (10 limbs, 8.0%).