During a 30-month period, 20 median sternotomy wound infections or endocarditis occurred after 20 of 1204 (1.7%) cardiac surgery procedures in adults at the University of Maryland Hospital. We examined four risk factors related to the individual undergoing surgery: age, sex, and index of obesity (weight/height), and presence of diabetes mellitus. The odds ratio estimates of the relative risk of infection observed with use of a population control group were as follows: sex (female) 3.5 (p less than 0.05), obesity 2.0, and presence of diabetes mellitus 3.8. For a second control group matched for age, type of operative procedure, and date of operation, the estimated relative risks of infection were sex (female) 2.1, obesity 6.2 (p less than 0.05), and diabetes mellitus 2.0. More precise definition of the relative risk of sternotomy infection associated with obesity and diabetes is required to aid surgeons and patients in making judgments about the relative benefits of surgery and to alert nursing personnel to be particularly aware of early signs of infection in patients at high risk.