A placebo-controlled study with progesterone compound, 17-alpha-hydroxyprogesterone 17-n-caproate (Primostat), in 39 patients with benign enlargement of the prostate is reported. Statistical analysis of the results showed no evidence of significant improvement in patients receiving hydroxyprogesterone-caproate. No evidence of an effect as compared with the placebo was found when the residual urine, prostatic size, and histologic and ultrastructural changes of the removed prostatic gland in 6 of the patients, and in the luteinizing hormone, follicle-stimulating hormone, and estrogen urine levels in 21 patients were examined. Subjective effects, when carefully analyzed, provided some beneficial evidence, however not substantiated, when the patients' mode of voiding was carefully watched. The reported beneficial subjective improvement might be attributed to the enhancement of the beta-adrenergic response by the progesterone compound of the adrenergic receptors in the posterior urethra and bladder, presumably causing relaxation of its smooth muscle. The problems associated with the choice and measurement of parameters to be used in this type of investigation are discussed, and the absolute necessity of proper controls, statistical analysis, and close follow-up of the patients is pointed out.