Slow transit constipation is notoriously difficult to treat. We tested whether polyethylene glycole 4000 (PEG) improves slow transit constipation. Eight female outpatients with symptoms of constipation and a colonic marker transit of more than 60 h were included (age 46 +/- 4y, duration of complaints 17 +/- 3y) in a randomised controlled cross-over study. During a 6-week placebo and 6-week PEG-phase (60g PEG/d) the following parameters were measured: 1. subjective well-being with respect to defeacation on a visual analogue scale (-8 cm very bad, +8 cm very good), 2. in the first 5 weeks of each phase, average dose of sodium picosulfate (prescribed as only allowed laxans, dose adjusted and protocolled on a diary by patient) 3. stool frequency, 4. colonic transit of radiopague polythene pellets in the last week of each phase (the use of laxative was strictly prohibited in this last week). Both subjective and objective parameters of colonic function improved significantly. Visual analog scale ratings improved from -4.65, [-8; 0.5] to 4.65, [-8; 7.5]cm (median, range) (p = 0.028), the self-administered dose of sodium picosulfate decreased from 4, [0; 37] to 0, [0; 11] drops per day (p = 0.028), stool frequency increased from 3.1, [1; 30] to 11, [2; 33] defeacations per week (p = 0.017), and total colonic transit decreased from 91, [67; 116] to 43 h, [17; 76]h (p = 0.017). In conclusion, PEG improves colonic function in patients with slow transit constipation subjectively and objectively. PEG should be considered as an additional option in patients refractory to established forms of treatment.