To determine whether or not implementation of an electronic medical record (EMR) in a primary care office is associated with an increase in completion of preventive care for a general population of adults and children, and for adults with diabetes mellitus. Observational cohort study that compared the change in completion rate of recommended interventions from before to one year after implementation of an EMR. The EMR was designed for direct interaction by providers and included guidelines for preventive care as well as automated prompts for when interventions were due. Patients were selected from a family medicine teaching practice in Delaware that implemented an EMR in July of 1998. We examined completion of recommended interventions for adults (influenza and pneumococcal immunizations, mammograms and cholesterol screening), children (hepatitis B and varicella immunizations) and adults with diabetes mellitus (monitoring for glycosylated hemoglobin and cholesterol, influenza and pneumococcal immunizations). Completion rates were compared from before to one year after EMR implementation. The number of persons analyzed ranged from 117 for diabetes interventions to 1148 for cholesterol screening. Completion rates increased from before to after EMR implementation for all outcomes studied. The largest increases were seen for mammograms (28.7 percent to 52.5 percent), varicella immunizations (29.6 percent to 55.9 percent), glycosylated hemoglobin (53.0 percent to 80.3 percent) and influenza immunization for persons with diabetes (29.7 percent to 55.1 percent). Implementation of a primary care EMR was associated with a substantial increase in completion of recommended preventive care across a broad spectrum of interventions and populations.