Epidemiologic studies have demonstrated that physical inactivity is an important determinant of numerous chronic diseases. However, self-reported estimates of physical activity contain measurement errors responsible for attenuating relative risk estimates. A validation study conducted in 2002–2003 at the Alberta Cancer Board (Canada) included a physical activity questionnaire, four 7-day physical activity logs, and four sets of accelerometer data from 154 study subjects (51% women) aged 35–65 years. The authors used a measurement error model to evaluate validity of the different types of physical activity assessment, and the attenuation factors, after taking into account error correlations between self-reported measurements. The validity coefficients, which express the correlation between measured and true exposure, were higher for accelerometers (0.81, 95% confidence interval (CI): 0.76, 0.85) compared with the physical activity log (0.57, 95% CI: 0.47, 0.66) and questionnaire measurements (0.26, 95% CI: 0.12, 0.40). The estimate of the attenuation factor for questionnaires was 0.13 (95% CI: 0.05, 0.23). Accuracy of physical activity questionnaire measurements was higher for men than for women, for younger individuals, and for those with a lower body mass index. Because the degree of attenuation in relative risk estimates is substantial, after the role of error correlations was considered, validation studies quantifying the impact of measurement errors on physical activity estimates are essential to evaluate the impact of physical inactivity on health.