We have previously reported an increase in forearm bone mineral content (BMC) during therapy for osteoporosis with the anabolic steroid, nandrolone decanoate. However, it has recently been claimed that part of this increase is spurious, due to a decrease in forearm fat during the treatment. We have therefore analyzed the data from a cross-over study of the effects of this agent on 70 osteoporotic women, using the fat correction procedure supplied by the manufacturer of the forearm densitometer. There was a significant rise (p less than 0.001) in the mean fat-corrected BMC (BMC[fc]) on nandrolone decanoate (50 mg intramuscularly every 2 or 3 weeks) and a non-significant fall in mean BMC[fc] off the drug. The mean time-weighted rate of change in the fat-corrected value was +29 +/- 5 mg/cm/year on nandrolone decanoate and -5 +/- 5 mg/cm/year off nandrolone decanoate (p less than 0.001). Nandrolone decanoate produces a significant gain in forearm mineral content even after allowing for changes in forearm fat content during therapy.