BACKGROUND Chronic constipation is frequent in children. The objective of this study is to compare the efficacy and safety of PEG 4000 and lactulose for the treatment of chronic constipation in young children. METHODS This randomised, double-blind study enrolled 88 young children aged 12 to 36 months, who were randomly assigned to receive lactulose (3.3 g per day) or PEG 4000 (8 g per day) for four weeks. The primary efficacy variable was stool frequency during the fourth week of treatment. Secondary outcomes were the number and frequency of subjective symptoms associated with defecation at each visit. RESULTS Stool frequency was comparable in the two groups at baseline (lactulose: 0.7 ± 0.5; PEG 4000: 0.5 ± 0.55). Mean stool frequency increased from 0.70 ± 0.50 stools/day at baseline to 0.80 ± 0.41 at Week 4 in the lactulose group and from 0.50 ± 0.55 to 1.10 ± 0.55 stools/day in the PEG 4000 group. A significant difference was observed in the adjusted mean change from baseline, which was 0.15 stools/day in the lactulose group and 0.51 stools/day in the PEG 4000 group, with a least-squares mean difference of 0.36 stools/day [95% CI: 0.16 to 0.56]. With respect to secondary outcome variables, stool consistency and ease of stool passage improved more in the PEG 4000 group (p = 0.001). The incidence of adverse events was similar in both groups, the majority of which were mild. CONCLUSIONS PEG 4000 has superior efficacy to lactulose for the treatment of chronic constipation in young children and is well tolerated. BACKGROUND US National Institute of Health Clinical Trials database; study NCT00255372 first registered 17th November 2005.