Quantitative estimation of hair growth using hair weight and number was recorded for 120 weeks in 4 groups of 9 men with androgenetic alopecia. Three double-blind groups applied either 2% or 5% minoxidil solution, or vehicle. The fourth group, unblinded, received no treatment. Measurements of hair weight and number were continued for 96 weeks, when treatment (if any) was stopped, though measurements were continued for another 24 weeks. Although not compared statistically, the placebo and untreated groups behaved in a similar fashion. In contrast, the 5% and 2% minoxidil treatment groups showed a statistically significant increase in mean percentage change in interval weight from baseline compared with placebo; results for number counts were usually less significant. Over 96 weeks, topical minoxidil induced and maintained an increase in interval weight over baseline of about 30%. After treatment was stopped, hair weight and number counts for the minoxidil groups returned to about the same levels as placebo in 24 weeks.