Melanosis coli was diagnosed histologically in colon biopsies of 45 patients with prolonged administration of anthraquinone laxatives. Colonoscopies performed for increased constipation, abdominal pains, or distention disclosed discoloration of the mucosa in only 14 patients. Radiographic studies revealed motility disorders of the colon. In several cases, a circular stenosis occurred at the junction of the colon and the sigmoid, simulating a tumor. Electron microscopy showed abnormalities of the absorptive epithelial cells. The lamina propria contained pigment-laden macrophages, plasma cells, and several nerve fibers in different stages of degeneration. The most striking changes of autonomic nerve elements occurred in patients with the most serious motility disorders.