To evaluate the evidence for the role of weight control and physical activity in cancer prevention and to identify priorities for research and for public health action in relation to the primary prevention of cancer, an international working group of experts was convened in Lyon in February 2001 by the International Agency for Research on Cancer of the World Health Organization. The expert group concluded that limiting weight gain during adult life, thereby avoiding overweight and obesity, reduces the risk of postmenopausal breast cancer and cancers of the colon, endometrium, kidney (renal cell) and esophagus (adenocarcinoma). Limiting weight gain possibly reduces risk of cancer of the thyroid. Weight loss among overweight or obese persons possibly reduces risks of these cancers, but no definite conclusion can be drawn because of the paucity of the epidemiological evidence. The working group also concluded that there was sufficient evidence for the role of physical activity in preventing colon and breast cancers, and limited evidence for the cancers of the prostate and endometrium. Some of these effects were independent of that of the weight control. Taken together, the working group considered that excess body weight and physical inactivity account for approximately a quarter to one-third of cancers of the colon, breast, endometrium, kidney (renal cell) and esophagus (adenocarcinoma). Thus adiposity and physical inactivity appear to be the most important avoidable causes of these cancers.