Vertical sections of small scalp biopsy specimens are often inadequate for the diagnosis of male pattern androgenetic alopecia (MPAA). Quantitative analysis of follicular structures in horizontal sections can provide more information. Our purpose was to establish better diagnostic criteria by comparing horizontal and vertical sections of scalp biopsy specimens from MPAA and normal control subjects and to determine the predictive value of horizontal sections, by relating counts of follicular structures in MPAA to subsequent hair regrowth from topical minoxidil therapy. Paired 4 mm punch biopsy specimens were taken from 22 normal control subjects and 106 patients with MPAA, for horizontal and vertical sectioning. In horizontal sections, hair bulbs, terminal anagen, catagen and telogen hairs, telogen germinal units, and vellus hairs were counted, as were follicular units and stelae. The diagnosis of MPAA was confirmed by finding decreased terminal hairs and increased stelae and vellus hairs. The average horizontal section contained 22 terminal and 13 vellus hairs, a 1.7:1 ratio. Changes compatible with MPAA were found in most vertical and horizontal sections, but horizontal sections were required for follicular counts and showed terminal:vellus hair ratios diagnostic of MPAA in 67% of cases. Of 44 patients treated with topical minoxidil, five with less than 2 follicular structures/mm2 showed no hair regrowth, 32 with 2 to 4 follicular structures/mm2 showed regrowth in 72%, and seven with more than 4 follicular structures/mm2 showed regrowth in 86% of cases. In MPAA with no significant inflammation, regrowth occurred in 77% of cases, versus 55% in cases with significant inflammation. Horizontal sections of scalp biopsy specimens in MPAA provide more diagnostic information than vertical sections and appear to have a predictive value for hair regrowth.