A 24-month clinical trial, begun on a double-blind basis, was conducted in 153 men with discernible male pattern baldness of the crown with the use of either topically applied placebo, 2% minoxidil, or 3% minoxidil solution. After 4 months the patients using placebo were switched to 3% minoxidil solution. At 12 months, there were statistically significant increases in terminal hair growth within a 1-inch target area in those treated with 2% or 3% minoxidil solution, in comparison with baseline counts. However, there were few patients who had appreciable cosmetic restoration. At 12- and 24-month intervals, progressive regression or stabilization of the size of the bald area was noted in the majority of patients. This therapeutic or preventive effect was statistically significant. The data on actual target area hair counts suggested that the 2% minoxidil solution was equal to or more efficacious than the 3% minoxidil solution. Baseline vital signs and laboratory parameters remained essentially unchanged. Topical minoxidil was well tolerated, with no serious drug-related adverse reactions noted during the study.