Assessment of constipation in childhood is difficult, particularly when the presenting symptom is spurious diarrhoea or faecal incontinence. We have therefore assessed the clinical usefulness, reliability and acceptibility of a solid marker transit technique in 52 patients with constipation (median age 8.0 years; range 2–13.5 years) at two referral centres. Median duration of symptoms was 60 months. Soiling was a prominent feature in 43 children (83%). Ten, 3 mm pieces of 6FG radio-opaque Silastic tubing were given orally at 9am on days 1, 2 and 3 and a plain abdominal film taken on day 5. Laxative treatment was not interrupted. Each film was divided into right colon, left colon and rectosigmoid areas, using bony landmarks, and the marker content of each area counted. The coefficient of variation of intra and inter-observer errors was 3.1% and 2.1% respectively. By day 5, 7% (group median) of markers were still in the right colon, 17% in the left colon and 42% in the rectosigmoid. Twenty-one patients(40%) had normal transit, 4 (8%) mild delay, 9 (17%) moderate and 18 (35%) severe transit delay. Marker distribution indicated slow pancolonic transit in 29% and slow segmental transit in 10%. In 21%, clustering of markers in the rectosigmoid suggested outlet obstruction. A significant correlation was found between both transit delay and marker distribution and the severity of clinical symptoms of constipation and soiling. Repeat studies in six children following colonic evacuation revealed significant improvement (P< 0.05) in marker transit. The test was well tolerated and was useful in establishing the diagnosis of constipation in children with soiling or spurious diarrhoea and in assessment of its severity and its response to treatment.