Fifty-eight men with Hamilton scale type III vertex or type IV male pattern baldness were studied to determine the dose-response activity of low concentrations of topical minoxidil in promoting hair growth. The patients were treated with topical minoxidil at doses of 0.01%, 0.1%, 1%, or 2% or placebo in a randomized double-blind design for 6 months. At the end of 6 months, patients using 0.1%, 1%, and 2% topical minoxidil solutions showed a significantly greater difference in the mean increase of nonvellus hair growth in comparison with those using 0.01% minoxidil or placebo. There was a clear dose-response correlation for the increase of nonvellus hairs in the 0.1%, 1%, and 2% minoxidil treatment groups. There was a statistically significant difference in patient's self-evaluation of overall hair growth and degree of decreased hair shedding in the 1% and 2% minoxidil groups when compared with the other study groups. From this study we conclude that significant increases in nonvellus hair counts occur with 0.1% and greater doses of minoxidil. However, only in patients treated with the 1% and 2% solutions of minoxidil was there clinically perceptible hair growth.