Cowden's syndrome is either familial or sporadic and is associated with the predominantly postpubertal development of a variety of cutaneous, stromal and visceral neoplasms. The syndrome is associated with mutations of the PTEN gene and is closely related to Bannayan's syndrome in which macrocephaly and benign tumors, especially lipomas and hemangiomas are pathognomic. In PTEN knockout mice testicular tumors have been reported and for this reason we felt it prudent to examine the testes of our patients with genetically proven Cowden's syndrome. Seven of eight patients who underwent testicular ultrasound were found to have diffuse bilateral hyperechoic lesions. Four patients consented to testicular biopsy and on histological examination multiple foci of adipocytes were found within the testicular interstitium, with no evidence of dysplasia or preclinical malignancy. Immunohistochemical assessment of adipocytes suggested a stromal derivation without evidence of metaplasia from Leydig cells. In one case there was focal atrophy of seminiferous tubules, while in two others there was nodular periorchitis of the tunica albuginea. Biochemical evaluation of testicular function (luteinizing hormone, follicle-stimulating hormone, testosterone, sex hormone binding globulin and free androgen index), prostate-specific antigen and testicular tumor markers were normal, while seminal fluid analysis showed only minor abnormalities. The high incidence of testicular lipomatosis in our adult subjects suggests this to be an important diagnostic criterion for Cowden's syndrome.