Objectives: To estimate the suppressive effect of partially hydrolyzed guar gum (PHGG) on transitory diarrhea induced by ingestion of a sufficient amount of maltitol or lactitol in female subjects. Design: The first, the minimal dose level of maltitol and lactitol that would induce transitory diarrhea was estimated separately for each subject. Individual subject was administered a dose that increased by 5 g stepwise from 10 to 45 g until diarrhea was experienced. Thereafter, the suppressive effect on diarrhea was observed after each subject ingested a mixture of 5 g of PHGG and the minimal dose level of maltitol or lactitol. Setting: Laboratory of Public Health Nutrition, Department of Nutrition and Health Sciences, Siebold University of Nagasaki. Subjects: Thirty-four normal female subjects (21.370.9 years; 49.575.3 kg). Main outcome measurement: Incidence of diarrhea caused by the ingestion of maltitol or lactitol and the ratio of suppression achieved by adding PHGG for diarrhea. Results: The ingestion of amounts up to 45 g of maltitol, diarrhea caused in 29 of 34 subjects (85.3%), whereas the ingestion of lactitol caused diarrhea in 100%. The diarrhea owing to maltitol was improved in 10 of 28 subjects by the addition of 5 g of PHGG to minimal dose-induced diarrhea, and that owing to lactitol was in seven of 19 subjects. Adding 10 g of PHGG strongly suppressed the diarrhea caused by maltitol, and the cumulative ratio was 82.1% (23/28). Conclusion: The transitory diarrhea caused by the ingestion of maltitol or lactitol was clearly suppressed by the addition of PHGG. These results strongly suggest that diarrhea caused by the ingestion of a sufficient amount of non-digestible sugar substitute can be suppressed by the addition of dietary fiber.