BACKGROUND: The International Physical Activity Questionnaire (IPAQ) is commonly used in surveys but reliability and validity has not been established in the Danish population. METHODS: Among participants in the Danish Health Examination survey 2007-2008, 142 healthy participants (45% men) wore a unit that combined accelerometry and heart rate monitoring (Acc+HR) for 7 consecutive days and then completed the IPAQ. Background data were obtained from the survey. Physical activity energy expenditure (PAEE) and time in moderate, vigorous and sedentary intensity level were derived from the IPAQ and compared with estimates from Acc+HR using Spearman´s correlation coefficients and Bland-Altman plots. Repeatability of the IPAQ was also assessed. RESULTS: PAEE from the two methods was significantly positively correlated (0.28 and 0.49 for women and men, respectively). Men significantly overestimated PAEE by IPAQ (56.2 vs. 45.3 kJ/kg/day, IPAQ: Acc+HR, P