Little is known about the prevalence and risk factors for development of irritable bowel syndrome (IBS) in Japan. In the United States, it is reported that heredity and social learning contribute to the development of IBS. Our aims were (1) to estimate the prevalence of IBS, (2) to confirm that subjects with IBS are more likely to have parents with a history of bowel problems, (3) to confirm that gastroenteritis is a risk factor for IBS, and (4) to determine whether these two risk factors interact with psychological distress. Prevalence was estimated from a sample of 417 young adults seen for annual health screening examinations. To evaluate risk factors related to consulting physicians, the 46 subjects who fulfilled Rome II diagnostic criteria for IBS but denied ever having seen a physician about these symptoms (IBS non-consulters)were compared to the 317 subjects who did not meet the criteria for IBS (controls) and to a group of 56 patients diagnosed with IBS by gastroenterologists(IBS patients). All subjects completed the Gastrointestinal Symptoms Rating Scale, the State-Trait Anxiety Inventory, the Self-Rating Depression Scale, the Perceived Stress Scale, and the SF-36 quality of life scale. Fourteen and two-tenths percent(15.5% of females and 12.9% of males) of the community sample met the criteria for IBS diagnosis, of whom 22% consulted physicians. IBS patients and IBS nonconsulters were more likely than controls to have a parental history (33.9 vs.12.6% , P< 0.001, for patients and 26.1 vs. 12.6% , P< 0.01,for nonconsulters) and were more likely to report an infective history compared to controls (44.6 vs. 16.1% , P< 0.001, for patients and 32.6 vs. 16.1% , P< 0.01, for nonconsulters). Two-way analysis of variance showed that the parental history was associated with a significantly greater impact on symptoms of indigestion, diarrhea, constipation, state and trait anxiety, and the SF-36 scales for social functioning and role emotional and that an infective history was associated with a greater impact on bodily pain. Both a parental history of bowel problems and a history of acute gastroenteritis are significant risk factors for development of IBS in Japan, as reported for the United States. Moreover, patients with such a family history show more psychological distress than other patients. Little is known about the prevalence and risk factors for development of irritable bowel syndrome (IBS) in Japan. In the United States, it is reported that heredity and social learning contribute to the development of IBS. Our aims were (1) to estimate the prevalence of IBS, (2) to confirm that subjects with IBS are more likely to have parents with a history of bowel problems, (3) to confirm that gastroenteritis is a risk factor for IBS, and (4) to determine whether these two risk Factors interact with psychological distress. Prevalence was estimated from a sample of 417 young adults seen for annual health screening examinations. To evaluate risk factors related to consulting physicians, the 46 subjects who fulfilled Rome II diagnostic criteria for IBS but denied ever having seen a physician about these symptoms (IBS non-consulters) were compared to the 317 subjects who did not meet the criteria for IBS (controls) and to a group of 56 patients diagnosed with IBS by gastroenterologists (I BS patients). All subjects completed the Gastrointestinal Symptoms Rating Scale, the State-Trait Anxiety Inventory, the Self-Rating Depression Scale, the Perceived Stress Scale, and the SF-36 quality of life scale. Fourteen and two- tenths percent (15.5 % of females and 12.9% of males) of the community sample with the criteria for IBS diagnosis, of whom 22% consulted physicians. IBS patients and IBS nonconsulters were more likely than controls to have a parental history (33.9 vs. 12.6%, P <0.001 for patients and 26.1 vs. 12.6%, P <0.01, for nonconsulters) and were more likely to report an infective history compared to controls (44.6 vs. 16.1%, P <0.001 for patients and 32.6 vs. 16.1% , P <0.01, for nonconsulters). Two-way analysis of variance showed that the parental history was associated with a significantly greater impact on symptoms of indigestion, diarrhea, constipation, state and trait anxiety, and the SF-36 scales for social functioning and role emotional and that an infective hiboth a parental history of bowel problems and a history of acute gastroenteritis are significant risk factors for development of IBS in Japan, as reported for the United States. Moreover, patients with such a family history show more psychological distress than other patients.