A workshop to evaluate the evidence for the role of physical activity in cancer prevention and to identify priorities for action, particularly in relation to the primary prevention of cancer, was held by Cancer Care Ontario in March 2000. A review of the scientific evidence was commissioned and an expert panel convened to consider the review report and to make recommendations for public health, research and intervention. The panel concluded that evidence was convincing for the role of physical activity in preventing colon cancer; probable for breast cancer; possible for prostate cancer and insufficient for other sites. It is recommended that physical activity messages promoting at least 30 45 minutes of moderate to vigorous activity on most days of the week be included in primary prevention interventions for cancer. The panel recommended that future research on physical activity incorporate comprehensive assessments, including measures of the multiple dimensions and types of physical activity; biological mechanisms; and behavioural and population factors. Cancer Care Ontario will incorporate physical activity messages in its primary prevention programming around nutrition and health body weight.