A study of primary care physicians was conducted in the provinces of Quebec and New Brunswick, Canada, to ascertain their patterns of preventive practice with respect to cancer in four anatomic sites: breast, cervix, colon-rectum, and lung. The determinants of preventive practices among 552 fee-for-service physicians in both provinces are explored. Scales were created for the practice behaviors related to each type of cancer (dependent variables) and for knowledge and belief (independent variables). The content of these scales was delineated through factor analysis and their reliability assessed using Cronbach's alpha. Other variables were considered in the analysis, including continuing education activities, perceived barriers to prevention, and other sociodemographic and professional variables. Bivariate analysis and multivariate techniques were used. The explanatory factors were regrouped into cognitive, sociodemographic, and organizational determinants. Particular patterns were delineated for each cancer type. In a fee-for-service reimbursement setting without specific incentives for preventive practices, the creation of favorable organizational environments and the conveying of agreed-upon information to physicians are important ways of enhancing the integration of preventive activities into clinical practice. Identification of the sociodemographic determinants of preventive practices reveals the complexity of physicians' behaviors.