Endocrinological effects of prolactin on human breast cancer have not been documented. The study was to determine relationships of serum prolactin level to clinical patterns or behaviors of breast cancer by measuring the serum prolactin level with RIA method. The first blood sample for prolactin determination was taken in the early morning when a patient was in rest and fast. Then, blood samples were taken in series after an injection of 500μg of TRH for stimulation. Patients studied were 61, including 57 with primary breast cancer, 4 with recurrent breast cancer. Control group consisted of 10 healthy women. Results obtained were as follows: 1) Basal level of prolactin was higher in 22 out of 61 patients (36%), compared with that of controls, and there was no statistically significant difference in the incidence in the patient whose basal level was higher than the upper limit of the control, between the each stage as well as between the pre- and postmenopausal group. 2) All patients responded to TRH stimulation and, in approximately 90% of the patient, the prolactin level reached to peak in a period of 15 to 30 minutes after the stimulation, regardless of the clinical stage and nearly one half of the patient except in stage I showed an excessive response. However, the return to the basal level was apparently retarded and about 30% of the patients showed higher level even at 60 and 90 minutes after the TRH stimulation. 3) There was no stage difference in incidence in patients whose levels 15 and 30 minutes after the stimulation were significantly elevated. However, the incidence in patient whose levels were higher that of the upper limit of controls was higher in patients belonged to stage Ⅱ and Ⅲ, compared with that in stage Ⅰ. 4) No significant correlation existed between lymph node involvement and incidence of patients with high prolactin level in each stage groups. However, significant increase was present in the 15-minute and 30-minute levels of prolactin in patients with lymph node metastases, though there was no significant difference in accordance with extension of the metastasis. 5) In histological classification, scirrhus type showed higher basal levels as well as the levels after the stimulation, compared with those of papillotubular and medullotubular types.