This study, using the Large Truck Crash Causation Study (LTCCS), examined the relationship between the mechanical condition of heavy trucks and crash involvement. Two specific hypotheses were tested. First, trucks with defects and out-of-service (OOS) conditions were statistically more likely to be in the role of precipitating a crash than trucks with no defects or OOS conditions. Second, defects in specific systems, such as the brake system, were associated with crash roles in which those systems were primary mechanisms for avoiding the crash and physical mechanisms that link the vehicle defect with the crash role. Postcrash inspections showed that the condition of the trucks in the LTCCS was poor. Almost 55% of the vehicles had one or more mechanical violations. Almost 30% had at least one OOS condition. Of mechanical systems, violations in the brake system (36% of all) and the lighting system (19%) were the most frequent. Both driver and vehicle factors were found to contribute to crashes. A brake OOS condition increased the odds of the truck's being assigned the critical reason (a variable identifying the precipitating vehicle) by 1.8 times. Both hours of service violations and log OOS violations increased the odds by a larger amount—2.0 and 2.2 times respectively. In rear-end and cross-paths crashes, brake violations, especially violations related to adjustment, increased the odds of the truck's being the striking vehicle by 1.8 times.