Background: Recently, polyethylene glycol (PEG 3350) has been suggested as a good alternative laxative to lactulose as a treatment option in paediatric constipation. However, no large randomised controlled trials exist evaluating the efficacy of either laxative. Aims: To compare PEG 3350 (Transipeg: polyethylene glycol with electrolytes) with lactulose in paediatric constipation and evaluate clinical efficacy/side effects. Patients: One hundred patients (aged 6 months-15 years) with paediatric constipation were included in an eight week double blinded , randomised, controlled trial. Methods: After faecal disimpaction, patients < 6 years of age received PEG 3350 (2.95 g/sachet) or lactulose (6 g/sachet) while children ≥6 years started with 2 sachets/day. Primary outcome measures were: de fecation and encopresis frequency/week and successful treatment after eight weeks. Success was defined as a defecation frequency ≥3/week and encopresis ≤1 every two weeks. Secondary outcome measures were side effects after eight weeks of treatment. Results: A total of 91 patients (49 male) completed the study. A significant increase in defeca tion frequency (PEG 3350: 3 prev 7 post treatment/week; lactulose: 3 prev 6 post/week) and a significant decrease in encopresis frequency (PEG 3350: 10 prev 3 post/week; lactulose: 8 prev 3 post/week) was found in both groups (NS). However, success was significantly higher in the PEG group (56%) compared with the lactulose group (29%). PEG 3350 patients reported less abdominal pain, straining, and pain at defecation than children using lactulose. However, bad taste was reported significantly more often in the PEG group. Conclusions: PEG 3350 (0.26 (0.11) g/kg), compared with lactulose (0.66 (0.32) g/kg), provided a higher success rate with fewer side effects. PEG 3350 should be the laxative of first choice in childhood constipation. Background: Recently, polyethylene glycol (PEG 3350) has been suggested as a good alternative laxative to lactulose as a treatment option in pediatric constipation. However, no large randomized controlled trials exist evaluating the efficacy of either laxative. Aims: To compare PEG 3350 ( Transipeg: polyethylene glycol with electrolytes) with lactulose in pediatric constipation and evaluate clinical efficacy / side effects. Patients: One hundred patients (aged 6 months-15 years) with pediatric constipation were included in an eight week double blinded, randomized, controlled trial. Methods: After faecal disimpaction, patients <6 years of age received PEG 3350 (2.95 g / sachet) or lactulose (6 g / sachet) while children ≥ 6 years started with 2 sachets / day. Primary outcome measures were: de fecation and encopresis frequency / week and successful treatment after eight weeks. Success was defined as a defecation frequency ≥3 / week and encopresis ≤1 every two weeks. Secondary outcome measur Results: A total of 91 patients (49 male) completed the study. A significant increase in defecation frequency (PEG 3350: 3 prev 7 post treatment / week; lactulose: 3 prev 6 post / week) and a significant decrease in encopresis frequency (PEG 3350: 10 prev 3 post / week; lactulose: 8 prev 3 post / week) was found in both groups (NS). However, success was significantly higher in the PEG group 56%) compared with the lactulose group (29%). PEG 3350 patients reported less abdominal pain, straining, and pain at defecation than children using lactulose. However, bad taste was reported significantly more often in the PEG group. (0.26 (0.11) g / kg), compared with lactulose (0.66 (0.32) g / kg), provided a higher success rate with fewer side effects. PEG 3350 should be the laxative of first choice in childhood constipation.