Physical activity level and cardiorespiratory fitness are both inversely associated with the risk of cardiovascular diseases and with all-cause mortality. Physical activity questionnaires are often validated against objectively measured maximal oxygen uptake (Vo_2max). To validate a self-report physical activity questionnaire against Vo_2max and furthermore to establish whether a simple question on self-rated physical fitness could predict objectively measured Vo_2max. A total of 102 men and women aged between 35 and 65 years were recruited from an ongoing population-based intervention study, the Inter99 Study. Participants reported their self-rated fitness and daily physical activity using a new questionnaire based on metabolic equivalents (MET). Vo_2max (ml/kg per min) was determined using a graded bicycle test with increasing workload until exhaustion and with simultaneous measurement of breath-to-breath oxygen uptake in 15-s periods. Statistical analyses were performed by linear regression analyses using the self-reported physical activity level as an independent variable and Vo_2max (l/min) as an dependent variable, and with covariates sex, age and bodyweight. Data from 53 men and 47 women were analysed. The amount of daily vigorous activity (>6 MET) was significantly positively associated with Vo_2max (P=0.0001, R ² = 0.76), whereas the total amount of physical activity was not significantly associated with Vo_2max (P=0.098, R ² = 0.69). A significant trend across three groups of self-rated fitness in relation to Vo_2max (ml/kg per min) was found (P for trend <0.0001). The physical activity questionnaire has acceptable validity when compared with Vo_2max in adult men and women. Furthermore, a simple question on self-rated fitness seems to reflect measured Vo_2max objectively.