Repigmentation induced by oral photochemotherapy (8-MOP + UV-A) in four patients with vitiligo has been studied by histochemical and ultrastructural techniques. Hypertrophic melanocytes were demonstrated both in the middle and deep portions of the hair follicles in the centre of islands of repigmentation and also in the epidermis of the expanding repigmenting border. Mitosis of melanocytes was absent in these areas. Ultrastructural study showed that the melanocytes of repigmented areas were hyperactive. The melanosomes were larger than those of surrounding healthy skin, although the mode of packaging was unaltered. These observations suggest that melanocytes repigmenting vitiliginous skin under the influence of oral photochemotherapy are derived from a melanocytic reservoir localized in the hair follicles.