We conducted a survey of a representative sample of all primary care physicians in the province of Québec to ascertain their patterns of preventive practice with respect to cancer in four anatomical sites: breast, colon-rectum, cervix, and lung. A stratified random sample of 430 physicians in general practice was interviewed individually and weighted population estimates derived. Physicians report teaching breast self-examination to their patients (96 per cent), performing breast examination (99 per cent), taking pap tests routinely (91 per cent), and pursuing anti-smoking counseling (98 per cent). Very few of them report submitting their patients over 50 years of age to annual mammography (8 per cent) or checking for occult blood in stools in patients over 45 years of age (15 per cent). Many still use routine chest X-rays as an early detection measure of cancer of the lung (77 per cent); an estimated 41 per cent use sputum cytology for the same purpose. Preventive practices, when in-use, are carried out mainly in the context of major encounters with patients such as general check-ups. Less than 28 per cent of the population is estimated to be reached by this strategy for prevention. The unrealized potential for prevention through capitalizing on all encounters with primary care physicians is important, and should stimulate creative efforts to enhance preventive activities in medical practice.