Previous studies of trigger digits in children have been limited to gross morphology and light-microscopic histology. Nine children with 11 trigger thumbs formed a preliminary study group for electron-microscopic evaluation of tendon nodules and A-1 pulleys. This pathoanatomic investigation was not previously reported. Comparison was made with light-microscopic sections. Large amounts of mature collagen was observed. Fibroblasts with prominent rough endoplasmic reticulum were present. No degenerative or inflammatory changes were noted in either tendon or sheath. We believe that although the etiology of trigger digits is still uncertain, an infectious, inflammatory, or degenerative process is unlikely.