5 alpha-Dihydrotestosterone (DHT) and androstanediols (diols) have been measured in human prostate tissue. DHT levels in surgical specimens of prostate from 8 patients with BPH averaged 5.6 +/- 0.93 S.E. ng/g and were significantly greater than (P less than 0.01) values of 2.1 +/- 0.32 S.E. ng/g in 6 normal prostates obtained post-mortem from males less than 50 yrs old. Androstanediols averaged 2.3 +/- 0.35 S.E. ng/g in the BPH specimens compared to values of 10.2 +/- 2.4 S.E. ng/g in the normal prostates (P less than 0.01). This significantly higher (P less than 0.001) ratio of diols/DHT in the normal (5.1 +/- 0.93 S.E.) compared to the BPH prostate (0.45 +/- 0.08 S.E.) suggests that a decrease in 3-hydroxysteroid oxido-reductase, which converts DHT to diol, may be an important clue to the pathogenesis of BPH.