Summary.— Twenty scalp hairs were plucked from each of 131 Caucasoid women, and measurements of diameter, at 1 mm intervals, starting just above the root, were made for each hair. There was little variation in diameter along normal anagen hairs in any of the subjects. The mean anagen hair diameter in 58 women with diffuse hair thinning, of whom 44 were without detectable endocrine abnormality and 14 were clinically hypothyroid, significantly decreased with age. This appeared to be an accentuation of a normal trend, although this could not be statistically established in the control group of 73 women. At any age the mean thickness of hair in the subjects with diffuse thinning was less than in normal subjects of the same age. Normal subjects showed a symmetrical distribution of hair diameter, with a marked peak at 0.08 mm. In contrast, subjects with diffuse alopecia without detectable endocrine abnormality showed a wide spread of diameters, with 2 equal peaks at 0.04 mm and 0.06 mm The hypothyroid group showed a similar distribution, except that there was a larger proportion of hairs of the smaller diameters. The results suggest that, in the development of diffuse thinning, not all follicles behave identically, and that there may be more than one type of follicle in the human scalp.