♦ Background: Fructo-oligosaccharides (FOS) exhibit soluble-fiber properties that beneficially affect bowel function and relieve constipation. The effects of FOS supplementation on constipation and biochemical parameters were examined in elderly continuous ambulatory peritoneal dialysis (CAPD) patients. ♦ Methods: This randomized, double-blind, placebocontrolled, cross-over study was performed in elderly CAPD patients (5 males and 4 females) with chronic constipation. All subjects were randomly assigned to receive either 20 g FOS or placebo daily for 30 days. After a 14-day washout period, the patients were switched to the other substance for 1 more month. Before and after each treatment period, frequency of defecation, characteristics of feces, and colonic transit were evaluated. Biochemical parameters were also assessed. ♦ Results: Fructo-oligosaccharides significantly increased the frequency of defecation (10.5 ± 2.0 vs 6.2 ± 1.4 times per week, p < 0.005) and changed the feces' appearance from type 1 (nut-like) to type 4 (sausage-like). The colonic transit determined by geometric center (GC) was augmented after FOS supplementation (3.9 ± 0.3 vs 3.2 ± 0.4, p < 0.05). Fructo-oligosaccharides had no effects on biochemical parameters. Fructo-oligosaccharides caused mild discomforts which were well tolerated after dose adjustment. ♦ Conclusions: Fructo-oligosaccharide supplementation is effective, well tolerated, and can be an alternative to other laxatives in CAPD patients with constipation. Further studies are needed to better assess the biochemical effects of FOS in the chronic kidney disease population.