Serum testosterone (T) and dihydrotestosterone (DHT) were measured by a sensitive and reliable radioimmunoassay in 42 normal subjects and 33 age-matched patients with carcinoma of the prostate. The mean +/- s.e. for serum testosterone in normal subjects was 16.74 +/- 0.76nM and the corresponding value for patients with carcinoma was 20.94 +/- 1.48nM. Statistical analysis of the results showed a significant increase in T level in patients with carcinoma of the prostate (P less than 0.01). In contrast, there was no difference in DHT concentration between the two groups, values being 2.43 +/- 0.09 and 2.06 +/- 0.09nM for normal subjects and patients respectively. The means +/- s.e. for T/DHT ratio in controls and patients were 6.8 +/- 0.2 and 12.8 +/- 1.3 respectively. The difference was highly significant (P less than 0.001). The wide range of variation for T in patients with carcinoma would suggest that although mean T is higher in these patients, this measurement alone is of little practical value, whereas T/DHT ratio is a more reliable parameter in evaluating the androgen changes in these patients. The significance of these findings in relation to the aetiology of the disease is discussed.