In adult men referred with gynaecomastia, the prolonged plasma oestradiol (E2) response to hCG appears to be a useful tool in the diagnosis of feminizing Leydig cell tumour (LCT) of the testis. However, its specificity is uncertain. The hormonal status was therefore studied in 85 men with gynaecomastia including 17 with LCT, 52 with clinically isolated gynaecomastia and 16 hypogonadal men (13 with Klinefelter's syndrome and three with hypogonadotrophic hypogonadism). The data obtained after hCG (5000 IU i.m., n = 85) or GnRH (100 micrograms i.v., n = 70) administration were compared to those of a group of control men. Means of basal gonadotrophin and of gonadotrophin responses to GnRH were similar in LCT and controls but were significantly (P less than 0.001) higher than controls in patients with either Klinefelter's syndrome or isolated gynaecomastia. Half the patients with isolated gynaecomastia had normal gonadotrophin parameters (NGG) while half had significantly (P less than 0.001) higher levels (EGG) than controls, suggesting either primary testicular failure or androgen resistance. The T responses to hCG did not differentiate patients with LCT from either isolated gynaecomastia or controls. A prolonged E2 response (greater than 300 pmol/l on day 3 following hCG) was observed in all patients with LCT, in 10/21 EGG patients and in 3/13 patients with Klinefelter's syndrome. Such a response was observed in only one NGG patient. We conclude that a prolonged E2 response to hCG does not appear to be specific to LCT but that specificity is greatly improved (from 80 to 98%) when the gonadotrophin pattern (basal and/or responses to GnRH) is also taken into account.