Abstract OBJECTIVES: Clinical studies in constipated adult patients have shown that a 17- or 34-g daily dose of polyethylene glycol (PEG) 3350 (MiraLax) is safe and effective for the treatment of constipation, with the best efficacy seen in wk 2 of treatment. The purpose of this study was to determine an optimal dose of PEG to provide satisfactory relief of constipation within 24 h. METHODS: A total of 24 adult study subjects who met Rome II criteria for constipation were randomized in a double-blind, parallel pilot study to receive a single dose of placebo or PEG laxative at doses of 51, 68, or 85 g in 500 ml of flavored water. Over a 72-h period, subjects rated bowel movements (BM), completeness of evacuation, and satisfaction. RESULTS: The 68-g dose seemed to be most satisfactory. Five of six subjects had a BM within 24 h. The time to first BM was 14.8 h for 68 g versus 27.3 h for placebo ( p = NS). The time to second BM was 19.2 h versus 47.2 h for 68 g and placebo, respectively ( p = 0.003). Of the subjects receiving 68 g of PEG, 50% and 100% reported complete evacuation for the first and second BM, respectively. The average number of BMs in 24 h for placebo, 51 g, 68 g, and 84 g were 0.5, 2.2, 2.2, and 4.2, respectively ( p = 0.004). There were no adverse reactions, and no patient reported incontinence or complained of cramps or diarrhea at any dose. There were no changes in measured electrolytes, calcium, glucose, BUN, creatinine, or serum osmolality. CONCLUSIONS: A 68-g dose of PEG laxative seems to provide safe and effective relief in constipated adults within a 24-h period.