We examined the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the growth and differentiation of cultured human hair outer root sheath cells (ORSC) from normal subjects and patients with vitamin D-dependent rickets type II (DDR-II) with alopecia. 1,25(OH)2D3 dose-dependently suppressed the plating efficiency, clonal growth, and DNA synthesis of normal ORSC. It enhanced the cornified envelope formation and caused morphological changes in the cells. All results indicated the existence of specific receptors for 1,25(OH)2D3 in the ORSC, and suggest that 1,25(OH)2D3 is a potent inhibitor of proliferation of ORSC as well as a stimulator of terminal differentiation. However, the cells from DDR-II patients with alopecia did not respond to 1,25(OH)2D3, suggesting a lack of the specific receptors in the cells. The differences in the cellular response to the hormone between the normal ORSC and those from the patients were apparent and easily distinguishable, therefore this experiment may be a rapid and simple diagnostic test for DDR-II patients with alopecia. Large number of hairs were difficult to obtain from patients with alopecia, and we developed a new culture method to accomplish these studies from a few plucked hair follicles. Our system may be useful in the culture of ORSC from limited number of follicles, and could be utilized to analyse the cellular characteristics of ORSC in patients with hair diseases.