Cells with the characteristic features of carcinoma in situ (CIS) were detected in histological sections of the gonads of three of seven children with clinical syndromes associated with a high risk of gonadal neoplasia. It is suggested that early detection of these cells may be useful in resolving problems of management, including gonadectomy and decisions about sex of rearing. Early detection of CIS cells may have a place in the management of the undescended testis.