The diagnosis and management of 22 patients with true hermaphroditism are described. Sixteen of them were first seen before the age of 4 months. The initial manifestations were ambiguous genitalia in 20 cases (two of them identified prenatally by ultrasound examination), isolated clitoromegaly in one, and penile hypospadias plus unilateral cryptorchidism in one. All patients but one had at least one palpable gonad. Eleven of the twelve patients examined before the age of 6 months had basal plasma testosterone levels > 0.4 ng/ml. In older patients the stimulation test was necessary to demonstrate male testosterone secretion. The most common peripheral karyotype was 46,XX (17 cases); the other karyotypes were 47,XXY (1 case) and mosaicism 46,XX/46,XY (2 cases) or 46,XX/47,XXY (2 cases). One of the patients with the 46,XX karyotype had 46,XX/46,XY on fibroblast culture; four had the SRY gene in their leukocytes and one in the tissue taken at gonadal biopsy. A vagina was found in all patients at laparotomy, and a uterus was found in 17 cases (as a hemiuterus in 9). Genitography failed to demonstrate a uterus in only one case. The testicular tissue was dysgenetic but the ovarian tissue was normal. Sex assignment was male in 8 patients (reoriented by us in 2) and female in 14 patients (reoriented by us in 3). Spontaneous pubertal development occurred in the 4 patients (2 boys, 2 girls) with gonadal tissue who reached pubertal age. We conclude that true hermaphroditism is a heterogeneous condition in terms of its genetic background, with a prevalence of the 46,XX karyotype. There may be mosaicism with a Y-bearing cell line limited to the gonad (its frequency is probably underestimated), a paternal meiotic exchange between X and Y occurring in 46,XX cases with SRY, or a lack of the SRY gene, suggesting that other genes working independently of SRY may also determine testicular differentiation.