OBJECTIVES: To estimate the prevalence of IBS-D in a US population, and to evaluate patients' self-reported severity of episodes. Prior use of medications commonly used to alleviate IBS symptoms and FS were also evaluated. METHODS: A two-stage, population based, cross-sectional survey was conducted from December 2001 - February 2002. Data were collected from U.S. participants via internet panel. Of the 31,829 person sampling frame, 25,986 (82%) completed a screening questionnaire to identify IBS and subtypes using ROME II criteria. A questionnaire was administered to IBS-D and alternating IBS (IBS-A) cases to assess symptoms, medication history, and FS using the Medical Outcomes Survey SF-36 instrument. RESULTS: Screening identified 1713 cases (6.6%) meeting Rome II criteria. Among these, 1,380 cases had IBS-D (n = 901) and IBS-A (n = 453). A total of 1180 of the 1380 IBS-D and IBS-A cases completed questionnaires. Between 8 - 43% of the cases' most recent episode were considered severe depending upon various definitions of severity. Over 95% of cases reported taking OTC medications, while 50% used prescription medications to treat IBS symptoms. Functional status scores of respondents were substantially lower than national norms on all dimensions of the SF-36. Individuals reporting their most recent episode as severe had still further impaired levels of functioning. CONCLUSIONS: This novel methodology and technology generated population-based estimates of IBS-D in a short period of time and assisted a drug sponsor in developing a risk management plan. The proportion of survey respondents defined as severe based on characteristics of their most recent episode is in contrast to estimated severity rates (~5%) in all IBS subtypes, based on physician assessments. This is the first study to evaluate FS in IBS at the population level. Using this methodology, additional questions may be posed to this IBS population.