Snapping thumb of the infant is usually caused by the disturbance of movement of the flexor tendon in its sheath. Few reports have dealt in detail with the histopathology of the snapping thumb. In the present study, light and electron microscopic observations were carried out on parts of the tendon and tendon sheath excised from snapping thumbs.The tendons and tendon sheaths were obtained from 21 fingers of 19 infants operated on over the last 2 years, under the diagnosis of snapping thumb. There were 10 males and 9 females, ranging in age from 1 to 6 years old. Sectioned samples were studied histochemically, using periodic-acid Schiff, toluidine-blue and high-iron-diamine alcian blue stains. The ultrastructure of the tendon nodule was also studied by scanning and transmission electron microscopy.Increase of the amorphous material and collagen fibrils was observed at the site of tendon nodule formation. The amorphous material was basophilic with hematoxylin-eosin staining, metachromatic with toluidine blue staining, and became dark violet with high-iron-diamine alcian blue, which indicates that it contains acidic mucopolysaccharide with a sulfate radical. The tendon sheath did not contain much amorphous material, but connective tissue cells proliferated. Synovial lining cells were not stratified and no inflammatory cells were found. By scanning electron microscopy, bundles of collagen fibrils of 0.1-0.15μ in diameter, probably newly synthesized, were observed in the middle portion of the tendon nodule. Transmission electron microscopy confirmed that ruthenium-red positive materials exist inside and outside of the fibroblasts. These fibroblasts seem to very actively produce acid mucopolysaccharide and collagen fibrils. Accumulation of the mucopolysaccharide and collagen fibrils by metaplasia of the fibroblasts in the endotenon and epitenon might make the nodule in the tendon. This tendon nodule formation is the primary cause of the snapping thumb of infants.