OBJECTIVES: LOGIC (Longitudinal Outcomes study of Gastrointestinal symptoms in Canada) and ILOS (Irritable bowel syndrome Longitudinal Outcomes Study) are two ongoing pharmacoepidemiologic studies of patients with irritable bowel syndrome with constipation or alternating constipation/diarrhea (IBS-C or IBS-A) across Canada and the US, respectively. The purpose was to develop and validate a self-report questionnaire that measures severity of IBS symptoms in patients with IBS-C and IBS-A. METHODS: The IBS patient's symptoms questionnaire was a 15-item questionnaire that measured frequency, intensity, and distress/bother associated with the following IBS symptoms: constipation, gas, abdominal pain/discomfort, bloating, and diarrhea. All items were measured on a 0 to 5-point scale except for the frequency items from the ILOS study, which were measured on a 0 to 4-point scale. To explore the questionnaire items, a principal component analysis (PCA) was performed. Correlations between the baseline severity scores and the baseline IBS-QOL overall scores were calculated. RESULTS: To date, 1557 subjects have provided the IBS severity of symptoms questionnaire at baseline for LOGIC. The PCA showed that three components provide a good summary of the data, with the 1st, 2nd, and 3rd components accounting for 37%, 20%, and 13% of the total variability in the data, respectively. Similar PCA results were observed for the 380 subjects who provided the ILOS questionnaire. For both studies, the first component approximates a simple average of scores for items of all symptoms excluding diarrhea. The resulting Severity Score significantly correlates with the IBS-QOL total score with r=–0.549 (p<0.001) for LOGIC and r=–0.468 (p<0.001) for ILOS. CONCLUSIONS: Although more extensive testing of the questionnaire is required, the IBS Severity questionnaire seems to efficiently and comprehensively measure patient severity of IBS symptoms. The Severity Score appears to be an effective measure of overall severity for subjects with IBS-C or IBS-A.