OBJECTIVES:  Health conditions including epilepsy, multiple sclerosis, cardiovascular diseases, diabetes, and psychiatric disorders are known risk factors for motor-vehicle accidents (MVAs). Yet, few studies have investigated if the co-occurrence of such conditions compounds the risk of MVAs. We examined the relationship between multimorbidity and MVAs with additional stratification by age and sex. METHODS:  The Cerner Health Facts® U.S. electronic health record database [2002 to 2012] was used to identify driver-related MVA cases in persons 16 years and older. Cases were matched to controls [1:5] receiving care from the same health facility on age and sex. Multimorbidity assessment was based on the prevalence of 13 predefined common health conditions retrospectively assessed over 2 years of patient history. MVA risk for individuals with increasing multimorbidty [1, 2, 3, 4, 5, 6+ conditions], relative to no morbidity, was estimated using conditional logistic regression adjusting for confounders that included alcohol abuse and socioeconomic status. RESULTS:  The 74,167 MVA cases identified were matched to 370,835 controls. Males (59.1%) and those under 44 years of age (68.2%) were more frequently involved in MVAs than their counterparts. Multimorbidity, 2 or more health conditions, was more common in cases (8.0%) than controls (5.3%), χ (1, N = 445,002) = 585.9, p<.0001. The relationship between multimorbidity and MVAs followed a positive monotonic dose–response curve. Compared to no morbidity, adjusted odds ratios increased from 1.12 (95% CI 1.07-1.16 ) to 1.37 (95% CI 1.23-1.52) to 3.65 (95% CI 2.78-4.79) with two, four, and six or more concurrent health conditions, respectively. The latter relationship was duplicated in the stratified analyses by sex and age. CONCLUSIONS:  Greater multimorbidity was associated with intensifying MVA risk. Given the demographic change involving population aging and its associated increase in multimorbidity, results of this study should be replicated in a population controlled for driving exposure.