Forty-seven men with male pattern baldness were treated in a double-blind clinical trial with topical 2% minoxidil or placebo. Twelve were randomly selected for quantitative hair measurement using the unit area trichogram and visual counting. There was no significant difference after 6 or 12 months of treatment with a 2% minoxidil solution for total hair density (THD; hair cm-2), meaningful hair density (MHD; hair greater than 40 microns in diameter greater than 30 mm in length cm-2), per cent of hair in the anagen growth phase, or the per cent of meaningful hair in the anagen growth phase. Significantly fewer hairs were recorded with the visual hair counting method, compared to values obtained from adjacent sites with the unit area trichogram. In addition, a significantly larger mean total hair count was recorded by an experienced observer, compared to an inexperienced observer. Increased pigmentation was observed within the vellus hair population of treated subjects. Our findings indicate that minoxidil appears unlikely to affect the long-term course of male pattern baldness. However, we found no significant deterioration in total hair density, or meaningful hair density in treated subjects, suggesting minoxidil may have a prophylactic effect. Further long-term studies employing the unit area trichogram are required to evaluate this finding.