The pathogenesis of hair loss, the postulated mechanisms of minoxidil action on hair growth, and clinical trials, adverse reactions, experimental formulations, and percutaneous absorption of topical minoxidil preparations are reviewed. Topical minoxidil seems to normalize hair follicles and increase blood flow to the scalp. In clinical trials of various formulations, results have varied. Improved hair growth occurred after four to six months of therapy; twice-daily application seems to be indicated. The most frequently reported adverse reactions are mild scalp dryness and irritation and, rarely, allergic contact dermatitis. Current recommendations are to reserve topical minoxidil for patients with normal cardiovascular status and to routinely monitor blood pressure, heart rate, and electrocardiographic changes. A new drug application is pending with FDA for use of topical minoxidil in androgenetic alopecia (male-pattern baldness), which is genetically determined and apparently stimulated by androgens. For alopecia areata, which involves hair loss on the body or scalp, usually patchy and of sudden onset, no reliable treatment has been found, although minoxidil may be efficacious in some patients. Minoxidil has generated new interest in hair-loss research. The etiology of hair loss must be better understood before more effective treatment regimens can be designed.