OBJECTIVES: To compare the accuracy of two absenteeism measures (days missed and hours missed) for inclusion in economic evaluations of IBS. METHODS: 135 IBS patients recruited from five US gastroenterology practices completed the Work Productivity and Activity Impairment questionnaire for IBS (WPAI:IBS), which assesses absenteeism with hours missed, and also completed questions about days and partial days missed. Days and partial days were considered to be the equivalent of 8 hours and 4 hours, respectively. RESULTS: 125 patients were employed, had complete data, and were included in the analysis. A total of 6.4% of patients reported missing days from work due to IBS in the prior 7 days, with an overall absenteeism rate of 1.8%; 28.0% of patients reported missing hours, with an overall absenteeism rate of 4.3%. Among those reporting hours missed, 42.9% missed less than 3 hours, 34.3% missed 3 to 5 hours, and 22.9% missed 8 to 30 hours. When days missed was used to measure absenteeism, 77.2% of patients missing hours and 60.7% of time missed due to IBS were not counted; when hours missed was reported, all patients reporting days missed were counted. Combining partial days missed with days missed increased the correspondence between those reporting days and hours missed, but considerably overstated absenteeism because partial-day absences were often less than 4 hours. Previous validation of the WPAI:IBS hours missed measure of absenteeism relative to measures of disease severity, verbatim responses and retrospective diaries, corroborates the inaccuracy of the days missed measure. CONCLUSIONS: Hours missed from work, not days missed, is a more accurate measure for capturing the partial-day absences characteristic of IBS patients. Other chronic disorders like IBS may exhibit a similar pattern of widespread absences of short duration, and that absenteeism may go undetected when days missed is the measure of absenteeism.