Gynaecomastia caused by Leydig cell tumours (LCT) in adult men may appear a long time before clinical evidence of testicular swelling. To evaluate the diagnostic criteria for LCT, hormonal status was studied in 14 cases and compared with results of a control group (CG) and 10 men with idiopathic gynaecomastia (IG). The mean plasma T level was significantly (P less than 0.005) lower in LCT (16.7 +/- 1.7 SEM nmol/l) than in CG (23.0 +/- 1.3 nmol/l). However, individual plasma T levels were in the normal range in 9/14 LCT. The mean plasma E2 level was significantly (P less than 0.001) higher in LCT (204.9 +/- 27.6 pmol/l) than in CG (87.9 +/- 7.7 pmol/l). However, individual plasma E2 levels were in the normal range in 5/14 LCT. In LCT, neither means of basal gonadotrophin levels nor the gonadotrophin responses to LHRH were different from CG. The mean of the plasma T responses to hCG did not differ between LTC, CG and IG. However the mean of E2 peak responses appeared significantly (P less than 0.005) higher in LCT (735.3 +/- 103.4 pmol/l) than in CG (420.5 +/- 40.4 pmol/l). The mean of the E2 peak responses was significantly (P less than 0.001) lower in IG (196.5 +/- 33.4 pmol/l) than in CG. Likewise the mean of plasma E2 levels, measured on day three following hCG administration, remained significantly (P less than 0.001) higher in LCT (662 +/- 94 pmol/l) than either in CG (228 +/- 14 pmol/l) or in IG (158 +/- 25 pmol/l). On day 3 following hCG administration, there was no overlap in individual plasma E2 levels between either LCT and CG or LCT and IG. In all LCT, plasma beta-hCG levels were in the normal range. A testicular echogram, performed in 12 LCT, confirmed the presence of a palpable tumour in 10 and revealed an occult tumour in two cases. We conclude that normal plasma beta-hCG levels, a prolonged plasma E2 response to hCG and testicular echogram appear to be the best criteria for early diagnosis of LCT responsible for gynaecomastia in adult men.