The effect of epilepsy or diabetes mellitus on the risk of automobile accidents BACKGROUND. Previous studies of possible associations between chronic medical conditions and traffic safety have been inconsistent and subject to bias because of the incomplete identification of affected persons. Recent advances in the diagnosis and management of epilepsy and diabetes mellitus have improved the control of these disorders and suggest a need to reexamine the risk of traffic mishaps among patients with these conditions. METHODS. We conducted a population-based retrospective cohort study of 30,420 subjects 16 to 90 years of age, with and without epilepsy or diabetes mellitus. Subjects included all the licensed drivers in seven contiguous ZIP Code areas in which the Marshfield Clinic and St. Joseph's Hospital, Marshfield, Wisconsin, are the primary sources of medical care. Standardized rates of moving violations and accidents over a four-year period (1985 through 1988) were compared in affected and unaffected cohorts. RESULTS. Standardized mishap ratios for subjects with diabetes were 1.14 for all moving violations (P = 0.23) and 1.32 for accidents (P = 0.01); for subjects with epilepsy the ratios were 1.13 for moving violations (P = 0.26) and 1.33 for accidents (P = 0.04). CONCLUSIONS. We conclude that drivers with epilepsy or diabetes mellitus have slightly increased risks of traffic accidents as compared with unaffected persons. The increases in risk observed in our study were generally smaller than those in previous studies, and we believe they are not great enough to warrant further restrictions on driving privileges. 