Introduction. There is debate as to whether physical activity counteracts the adverse effect of weight on health outcomes. We investigated how physical activity modifies the effect of body mass index (BMI) on hypertension risk. Methods. BMI, physical activity, and hypertension were measured at baseline and at three-year interval for 14 years (from 1996 to 2010), in 10,339 participants in the Australian Longitudinal Study on Women’s Health. Generalised estimating equation models for binary repeated measures were performed to determine the individual and joint effects of BMI and physical activity on incident hypertension. Results. At baseline (mean age ？SD), 57% were healthy weight, 28% overweight, and 14% obese. Increasing BMI and decreasing physical activity were associated with increased risk of hypertension. Physical activity attenuated the positive association between weight and risk of hypertension, especially for obese women. Compared to healthy weight high active women, risk of hypertension in obese high active women was 3.4 times greater (OR 3.43, 95% CI 2.68, 4.39) and in obese inactive women 4.9 times greater (OR 4.91, 95% CI 3.92, 6.13). Conclusions. Both physical activity and maintenance of a healthy body weight are associated with lower risk of hypertension. Physical activity reduced but did not remove the effect of obesity on hypertension risk. 1. Introduction There is persuasive evidence that obesity increases [1], whilst regular physical activity reduces the risk of cardiovascular disease [2]. However, contradictory findings from previous studies has led to debate about the relative importance of weight and physical activity with respect to risk of future adverse health outcomes [3–5]. Two studies have shown that the risk of all-cause mortality among overweight but fit men was similar to [6] or less than [7] the risk among healthy weight unfit men, leading to the hypothesis that cardiovascular fitness may offset the health risks of being overweight. In contrast, physical activity, rather than cardiovascular fitness, has not been found to negate the excess mortality or cardiovascular risk resulting from being overweight or obese [8–12]. In keeping with this, there is evidence that physical activity also does not completely counteract the risk of diabetes among overweight or obese individuals [3]. However, little is known about the effect of physical activity on the association between weight and hypertension [5]. To our knowledge, only one previous prospective population-based study has addressed this issue [13]. In it, Hu et al. found
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