All autopsied car drivers (n=480) aged ≥ 18 years, who were fatally injured and died within three days of the crash in northern Sweden over a 13-year period were studied. A grading system was developed to assess the probability of contribution of intrinsic medical factors (IMF) to the crash. This system takes into account both the risk of sudden incapacitation (inability to cope), based on IMF revealed in the medical history and at autopsy, and extrinsic nonmedical contributing factors. Almost one quarter of the drivers were found to have IMF. Intrinsic medical factors were more common in males than in females. Drivers with IMF were often at fault and usually crossed over to the wrong side of the road and crashed into an oncoming vehicle or roadside object. In 6% of the drivers, IMF were probably the underlying cause of the crash;​ in 1.3% the probability was strong. Thus, medical impairment was probably an underlying cause in 1 of 17 fatal crashes (1 of 5 in the elderly).