This study examined the extent to which physician gender influences practice patterns. Data came from the Ontario Hospital Insurance Plan billing profiles of general practitioner and family medicine graduates of McMaster University School of Medicine. The women physicians studied were more likely to be certified in family medicine than the men and a higher proportion of their patients were female. Women were more likely to be working part time, billed during fewer months of the year, earned less, and saw fewer patients. They provided greater numbers of services in psychotherapy and counselling and ordered more laboratory tests; associated with this were higher costs per service and per patient. Women offered a less diverse mix of services than men. They provided fewer hospital, emergency room, and intrapartum services and a lower proportion of women included house calls, after-hours work, hospital, emergency room, surgical or intrapartum services in their service mix. Thus these women appeared more likely to restrict their practices to the office setting and to provide a higher proportion of psychosocial care. The overall impact of these sex differences in practice patterns on the health care system requires further exploration.