Background: Finasteride, a type II 5⍺-reductase inhibitor, reduces scalp and serum dihydrotestosterone and has been shown to be effective in men with androgenetic alopecia (AGA). Objective: The purpose of this study was to determine the effect of finasteride on scalp hair weight in men with AGA. Methods: Sixty-six men with AGA received finasteride, 1 mg/d, or placebo in a 48-week study, and 49 men continued in a 48-week extension. Efficacy was assessed by scalp hair weights and hair counts. Results: As expected, hair counts improved with finasteride (net mean percent change ± SE [95% CI] compared with placebo = 9.2% ± 2.8% [3.8, 14.6] and 15.4% ± 3.2% [9.1, 21.7] at 48 and 96 weeks, respectively; P < .01 for both time points), and net improvements in hair weight were greater (25.6% ± 3.6% [18.5, 32.7] and 35.8% ± 4.6% [26.7, 44.8] at 48 and 96 weeks, respectively; P < .001 for both time points). Finasteride was generally well tolerated. Conclusion: In this study, finasteride, 1 mg, increased hair weight in men with AGA. Hair weight increased to a larger extent than hair count, implying that factors other than the number of hairs, such as increased growth rate (length) and thickness of hairs, contribute to the beneficial effects of finasteride in treated men. (J Am Acad Dermatol 2002;46:517-23.)