Abstract As in most industrialized societies, cancer is the leading cause of death in Quebec: in 2013, it is estimated that 20,200 people will die of cancer and that 48,700 new cases will be diagnosed. Carcinogenesis is a complex, multifactorial process that begins many years before clinical symptoms of cancer appear. Cancer is considered to be occupational in origin when it results from workplace exposure to a chemical, physical or biological agent, or from conditions inherent in a work activity. The disease would probably not have developed if the person had not done that job. A number of studies have estimated that from 2% to 8% of all cases of cancer are attributable to work, depending on the country and the number of cancer sites and types considered. However, for some sites or types, the proportion of cases attributable to work is far higher: occupation is thought to account, for instance, for over 90% of all cases of pleural mesothelioma in men. To be able to prioritize research and prevention needs, an accurate picture of the extent of the problem is needed. The purpose of this report is therefore to present an estimate of the number of cases of occupational cancer among Quebec workers.The number of cases of workplace cancer was estimated by means of two complementary methods: first, using Quebec workers’ compensation data for cases of occupational cancer and, second, based on the published percentages for forms of cancer attributable to work. While these two data sources have certain limitations, they offer the advantage of being readily available and allowing forms of cancer to be classified for the purposes of setting research priorities and guiding prevention initiatives.According to cancer compensation data from the Commission de la santé et de la sécurité du travail (workers’ compensation board, or CSST), in Quebec, fewer than 100 workers were compensated annually between 2005 and 2007 for a new diagnosis of work-related cancer. In 362 of the files opened between 1997 and 2005, compensation was paid out to people who died of cancer, including six women (all from mesothelioma). For men, over half of cases were mesothelioma, of the pleura or the peritoneum, with the second leading cause being lung cancer. The majority of workers compensated had been employed in manufacturing, mining or construction. Owing in part to the difficulty of establishing a cause-and-effect relationship and also to the lengthy latency period between workplace exposure and development of the disease, compensation is paid out by the CSST in only a small number of cancer cases.The second method used to estimate the number of cases of occupational cancer consisted in applying the percentages of cancer cases attributable to work (published by researchers in places fairly similar to Quebec, such as Finland or the United Kingdom) to the total number of deaths from cancer and the number of new cases of cancer diagnosed each year in Quebec. This method indicates that 6% (between 5% and 8%) of all new cases of cancer could be work-related (8% to 13% for Quebec men and 2% to 3% for Quebec women), for a total of 1,800 to 3,000 new cases of cancer annually between 2002 and 2006. The forms of occupational cancer that affect the greatest number of Quebecers are cancer of the trachea, bronchus and lungs, prostate, skin (excluding melanoma) and bladder, along with mesothelioma, cancer of the colon and non-Hodgkin's lymphomas. The most common forms of cancer for Quebec women are breast cancer and cancer of the trachea, bronchus and lungs. It has also been estimated that 8% (between 7% and 11%) of all cancer deaths (11% to 17% among men and 2% to 4% among women) are work-related, which would correspond to 1,110 to 1,700 deaths a year from cancer between 2002 and 2006. The most common forms of cancer that lead to death are cancer of the trachea, bronchus and lungs for both sexes, followed by mesothelioma, colon and prostate cancer among men, and breast cancer among women.The above figures highlight the (underestimated) extent of the burden of occupational cancer in Quebec. We did not attempt to put a dollar figure on the socioeconomic impact of these diseases. An assessment of this kind was conducted in Alberta, however, using cancer data from 2006 and cost data from 2008. The researchers estimated that for 761 new cases of work-related cancer annually and 2,700 people living with some form of occupational cancer, $15.7 million was spent in direct medical costs and $64.1 million in indirect socioeconomic costs every year. The total burden must be at least as high in Quebec, where it is estimated that there are 1,800 to 3,000 new cases of work-related cancer annually.The long latency period between exposure to a carcinogen and the appearance of cancer symptoms means that cases being seen now are the result of exposure from 10 to 50 years ago. Yet Quebec workers are still being exposed to carcinogens, and now is the time to act if we want to have an impact on their health in the coming decades.