Obesity is a major public health problem associated with a wide range of health problems. This study estimates the prevalence of obesity, calculates the proportion (or population-attributable fraction [PAF]) of major chronic diseases which is attributable to obesity, estimates the deaths attributable to it and projects its future prevalence trends. In Canada, the overall age-standardized prevalence proportion of obesity has increased from 10 percent in 1970 to 23% in 2004 (8 percent to 23 percent in men and 13 percent to 22 percent in women). The increasing prevalence of obesity was observed for all five age groups examined: 20-34, 35-44, 45-54, 55-64 and 65+. On average, the PAF of prevalence of selected major chronic diseases which is attributable to obesity from 1970 to 2004 has increased by 138 percent for men and by 60 percent for women. Overall, in 2004, 45 percent of hypertension, 39 percent of type II diabetes, 35 percent of gallbladder disease, 23 percent of coronary artery diseases (CAD), 19 percent of osteoarthritis, 11 percent of stroke, 22 percent of endometrial cancer, 12 percent of postmenopausal breast cancer, and 10 percent of colon cancer could be attributed to obesity. In 2004, 8,414 (95 percent CI: 6,881-9,927) deaths were attributable to obesity. If current obesity prevalence trends remain unchanged, the prevalence proportion of obesity in Canada is projected to reach 27 percent in men and 24 percent in women by the year 2010. These increases will have a profound impact on the treatment needs and prevalence of a wide variety of chronic diseases, and also on the health care system in terms of capacity issues and resource allocation.