Anthranoid-containing laxatives--aloe, cascara, frangula, and rheum--may play a role in colorectal cancer. This risk is particularly important in view of the wide abuse of self administered laxatives for chronic constipation. There are data on the genotoxic potential of anthranoids and there is evidence of a tumourigenic potential in rodents. A case report and clinical-epidemiological studies have evaluated the cancer risk in patients who have abused anthranoid laxatives over a long period. Pseudomelanosis coli is a reliable parameter of chronic laxative abuse (> 9-12 months) and is specific for anthranoid drugs. In a retrospective study of 3049 patients who underwent diagnostic colorectal endoscopy the incidence of pseudomelanosis coli was 3.13% in patients without pathological changes. In those with colorectal adenomas, the incidence increased to 8.64% (p < 0.01), and in those with colorectal carcinomas it was 3.29%. This lower rate was probably caused by incomplete documentation of pseudomelanosis coli in those with carcinoma. In a prospective study of 1095 patients, the incidence of pseudomelanosis coli was 6.9% for patients with no abnormality seen on endoscopy, 9.8% (p = 0.068) for patients with adenomas, and 18.6% for patients with colorectal carcinomas. From these data a relative risk of 3.04 (1.18, 4.90; 95% confidence interval) can be calculated for colorectal cancer as a result of anthranoid laxative abuse.