PURPOSE: Melanosis coli, which is a relatively common disease encountered during colonoscopy, is a brownish or blackish pigmentation of the colon and is associated with the ingestion of anthraquinone compounds. Its histopathological pathogenesis is recognized as colonic epithelial apoptosis. This study was designed to identify the relation between melanosis coli and the colonic transit time, as well as the relation between melanosis coli and other clinical characteristics. METHODS: We reviewed the clinical records of 80 patients with melanosis coli who had been diagnosed by colonoscopy between Jan. 1997 and Sep. 1998. The colonic transit time was checked in 26 cases of patients with melanosis coli and in 28 cases of patients with constipation without melanosis coli by using the multiple marker bolus technique. RESULTS: 1) The frequency of melanosis coli in patients undergoing a total colonoscopy was 1.25% (80/6422). 2) The age distribution was 20 to 76 years old (mean: 49). The grade of melanosis coli seemed to be related with age. 3) The four causative agents related with melanosis coli were anthraquinone compounds (69.2%), tea (12.3%), herbs (12.3%) and health foods in that order. 4) The duration of medication was from 2 months to 20 years (mean: 48 months). The grade of melanosis coli seemed to be related with the duration of medication. 5) The colonic transit time was normal in 11 cases (42.3%) and delayed in 15 cases (57.5%) of melanosis coli and normal in 10 cases (35.7%) and delayed in 18 cases (64.3%) of patients with constipation without melanosicoli. 6) Colon polyps were detected in 12 cases (15%). However, no relation between the incidence of colon polyps and either the grade of melanosis coli or the duration of medication was noted. CONCLUSIONS: Melanosis coli is a marker of ingestion of some drugs, such as anthraquinone compounds, and/or some kinds of acidic polysaccharides. Melanosis coli is not an indicator of colonic motility disorder or a danger signal for colon polyps or cancer.