The Ontario Cancer Registry (OCR) contains information on the incidence, mortality and survival of cancer in Ontario. The OCR refers to the population of Ontario, which is currently more than 9 million people. Cancer registration is accomplished by the computerised linkage of records collected routinely for other purposes. Incidence data are available from the OCR beginning in 1964. Incidence rates, age-standardised to the World Standard Population, are presented for Ontario in two recent 5-year periods (1979-1983 and 1984-1988). Age-specific rates are also presented for selected sites. The most common cancer sites, in terms of Ontario incidence rates, were similar to those reported from other registries. In males, these were cancers of the lung, prostate, colon, bladder and rectum. In females, these were cancers of the breast, lung, colon, body of the uterus and ovary. The potential effects of changes in medical services, screening practices and risk factor prevalence on the incidence rates and trends are discussed.