To determine whether benzodiazepine tranquilizers increase the risk of accidental injury requiring medical attention, we used pharmacy claims submitted to a large third-party payer to identify 4,554 persons who had been prescribed these agents and a matched control group of 13,662 persons who had been prescribed drugs other than benzodiazepines. We then used diagnoses recorded on claims submitted by medical care providers to identify all accident-related care received by these persons during three months before their first-observed prescription for a benzodiazepine or nonbenzodiazepine agent, respectively, and six months subsequently. We found accident-related care was more likely among persons who had been prescribed benzodiazepines; among these persons, the probability of an accident-related medical encounter was higher during months in which a prescription for a benzodiazepine had recently been filled compared to other months; and persons who had filled three or more prescriptions for these agents in the six months following initiation of therapy had a significantly higher risk of an accident-related medical event than those who had filled only one such prescription. Approximately two-fold risks of accident-related care were found, after controlling for age, sex, and prior utilization.