A multicentre randomised clinical trial was performed to compare the therapeutic potential of osaterone acetate with that of delmadinone acetate in the treatment of benign prostatic hyperplasia in dogs. The osaterone was administered orally at 0·25 mg/kg bodyweight once a day for seven days to 73 dogs. The delmadinone was administered by a single intramuscular or subcutaneous injection at 3 mg/kg bodyweight to 69 dogs. During the 180-day trial, the dogs were monitored five times for their clinical signs and prostate volume. The two drugs were similarly effective in reducing the clinical signs and inducing complete clinical remission, and both induced a similar level of minor, mostly transitory adverse effects. Osaterone reduced the volume of the prostate glands of the dogs significantly more quickly than delmadinone.