A 16-year-old male with long-standing atrophic chronic lymphocytic thyroiditis was evaluated for macroorchidism. A testicular biopsy prior to treatment revealed peritubular and interstitial fibrosis, reduced spermatogenesis and sparse Leydig cells with nonprominent smooth endoplasmic reticulum. Biological/immunological LH and FSH ratios were reduced, I-LH and FSH response to GnRH was blunted, and levels of testosterone and androstenedione were low. Twenty-two months after thyroid treatment, the testicular size was unchanged, and the degree of fibrosis showed minimal regression. Spermatogenesis with normal morphology was present, Leydig cells with Reinke crystals were present, and surface area and diameter of the seminiferous tubules had increased only slightly. There was a normal I-LH and FSH response to GnRH, and normal levels of testosterone and androstenedione. This study, along with previous reports, suggests that the etiology of the hypothyroid state may influence the development of testicular fibrosis.