Primary faecal incontinence (encopresis) in children is usually treated with laxative medication and a behaviour modification programme aimed at promoting regular toileting, but the effectiveness of laxatives has never been adequately investigated. 169 children with encopresis and evidence of stool on plain abdominal radiograph were randomly allocated to receive multimodal (MM) therapy (laxatives plus behaviour modification; n = 83) or behaviour modification alone (BM; n = 86). Mean (SD) follow-up was 55.1 (27.0) weeks and 56.7 (32.0) weeks, respectively. By 12 months' follow-up 42 (51%) of the MM group and 31 (36%) of the BM group (p = 0.079) had achieved remission (at least one 4 week period with no soiling episodes) and 52 (63%) vs 37 (43%) (p = 0.016) had achieved at least partial remission (soiling no more than once a week). MM subjects achieved remission significantly sooner than BM subjects, and the difference in the Kaplan-Meier remission curves was most striking in the first 30 weeks of follow-up (p = 0.012). The patterns of compliance with toileting in the treatment groups were almost identical, although about 1 in 8 children overall did not comply with the sitting programme. After exclusion of the 24 poor compliers, there was no significant difference between BM and MM groups. This study shows a clear advantage overall for the use of laxative medication, although the benefit may not be as great for children who are able to maintain regular toileting.