Histological and ultrastructural study of an adult man with acquired immunodeficiency syndrome (AIDS) revealed multiple testicular lesions. The seminiferous tubules varied from dilated tubules with hypospermatogenesis, to tubules with Sertoli cells and a few spermatogonia, to necrotic tubules. The testicular interstitium showed abundant inflammatory infiltrates, some of them forming microabscesses. The Sertoli cells exhibited spherical intranuclear inclusions corresponding to both cytomegalovirus and sphaeridia. Some sphaeridia stained intensely with EDTA as granular and fibrillar portions of the nucleolus; however, many sphaeridia stained weakly as nucleolar fibrillar centers. These observations suggest that a Sertoli cell response to cytomegalovirus is the proliferation of nucleolar organizing centers that in some instances may transform to give rise to more or less abnormally developed nucleoli.