In patients with constipation the prevalence of melanosis in rectal biopsies was evaluated in an attempt to correlate its occurrence with laxative consumption and intestinal stasis. Melanosis was present in 58 percent of the patients and in none of a control group. Melanosis was present in 73.4 percent of patients consuming anthracene laxatives and in 26.6 percent of those not consuming anthracene laxatives (P less than 0.01). No correlation was found between the occurrence (and grading) of melanosis and pattern of transit through the large bowel, bowel movements, and duration of symptoms. Results of this study seem to indicate that intestinal stasis is not a cause of melanosis of the colon and rectum and confirm that melanosis may well be due only to the consumption of anthracene laxatives; melanosis coli does not appear to be a sensitive marker of impairment of motor function in the "cathartic colon."