The costs and medical benefits of an early, routine laxative screening test in patients with diarrhoea of uncertain origin was evaluated. During a two year period 200 consecutive, unselected patients complaining of diarrhoea were considered for the study in whom a three day faecal collection was undertaken. Fifty four patients denying laxative consumption had diarrhoea (mean daily stool weight > 200 g) of uncertain origin at their initial visit of whom 47 were screened to detect ingestion of anthraquinones, bisacodyl, phenolphthalein, and magnesium salts. Seven patients had positive tests. No single clinical feature could have predicted the outcome of the test. The possible cost savings of the programme were estimated by not releasing the results of the test to the clinicians until the patient's investigations were complete. The seven patients with laxative abuse spent a total of 35 days in hospital and were seen on 29 occasions in the outpatient clinic after the laxative screening test was positive. The cost of the screening programme was cheaper than the costs of the diagnostic procedures in patients with laxative abuse. We recommend the use of a comprehensive, early laxative screening programme in all patients with diarrhoea of uncertain origin as a cost effective procedure.