The unit area trichogram was defined in the frontal and occipital areas of ten normal men and ten normal women and the results compared with the findings in ten male and fifteen female patients with androgenic alopecia. The epilation technique provided accurate data of normal hair density (higher than previously reported) and the phases of hair growth. The measurement of hair diameter showed that of 7603 hairs, 1241 were less than 40 microns in diameter, of which only 21 were more than 80 mm in length. Variation in the diameter of individual hair fibres was recorded. The number of hairs less than 40 microns diameter per cm2 of scalp was increased in both men and women with androgenic alopecia. It was concluded that the number of hairs greater than 40 microns in diameter per cm2 (meaningful density) reflected most closely the amount of hair usefully contributing to an individual's clinical appearance.