Nine patients with extensive pityriasis alba were studied using histopathological and histochemical techniques and electron microscopy. There was a reduction in the density of functional melanocytes in the affected areas without any change in their cytoplasmic activity. The melanosomes tended to be fewer and smaller, but their distribution pattern in the keratinocytes was normal. Melanosomal transfer to keratinocytes was generally not disturbed. The histology was non-specific. Hyperkeratosis and parakeratosis were not consistently present, and it seems unlikely that they played a significant role in the pathogenesis of the hypomelanosis. A variable degree of intercellular oedema and intracytoplasmic lipid droplets were present. The hypopigmentation may thus be due primarily to the reduced numbers of active melanocytes and a decrease in number and size of melanosomes in the affected skin.