BACKGROUND: To examine seasonal variations in self-reported physical activity among an urban population of Calgarian adults. METHOD: Telephone surveys were conducted with two independent random cross-sectional samples of adults in summer and autumn 2007 (n=2199) and in winter and spring 2008 (n=2223). Participation and duration of walking for recreation (WR), walking for transportation (WT), moderate (MODPA) and vigorous physical activity (VIGPA) undertaken in a usual week were captured. Seasonal comparisons of participation related to these activities and sufficient MODPA (≥210 min/week) and VIGPA (≥90 min/week) physical activity were examined using logistic regression. RESULTS: Compared with winter, participation in WR was significantly (p<0.05) more likely in summer (OR 1.42), autumn (OR 1.35) and spring (OR 1.40), WT was more likely in autumn (OR 1.27), and MODPA was more likely in summer (OR 1.42). Achievement of sufficient MODPA was significantly more likely in summer (OR 1.80), autumn (OR 1.31) and spring (OR 1.24). Although there was no seasonal variation in sufficient VIGPA overall, variations in seasonal pattern among sub-populations were observed. Sex- and age-specific seasonal patterns in physical activity were also found. CONCLUSION: Measuring physical activity throughout the year, rather than at one time point, would more accurately monitor physical activity and assist in developing seasonally appropriate physical activity interventions. Moreover, in countries that experience extreme weather conditions, creating physical activity-friendly environments that help overcome these conditions might contribute to year-long physical activity participation.