A case-control study examining the 1-yr motor vehicle accident experiences of 158 insulin-dependent diabetes mellitus (IDDM) cases and 158 nondiabetic siblings was undertaken to evaluate the risk of motor vehicle accidents among drivers with IDDM. In multivariate analyses the overall accident risk of the cases and control subjects did not differ significantly. Female diabetic drivers, however, showed a marked increased risk for motor vehicle accidents. The accident risk among female cases was five times higher than among the female control subjects (P less than .05). Age and marital status were also significantly associated with accident probability in the multivariate model. The results suggest that IDDM could have an effect on the accident rate of diabetic drivers, particularly women. However, the traditional risk factors for automobile accidents, i.e., age and marital status, appear to have an equally strong influence on accident occurrence. Further studies are needed to 1) document the role of IDDM in accidents among representative samples of the IDDM population and 2) properly evaluate the licensing restrictions recommended for diabetic drivers.