It is well known that driving under the influence of alcohol increases the risk of traffic accidents. Alcohol-dependent patients are responsible for two-thirds of motor vehicle crashes involving alcohol. Studies carried out on the general population have shown a relation between alcohol dependence and traffic accidents. The aim of the study is to analyse the effect on traffic accidents of treatment of patients with alcohol-related problems. To do so, the prevalence of traffic problems in a sample of patients with a diagnosed dependence on alcohol was assessed for three periods: during their lifetime, in the year preceding the start of treatment and over the year of treatment. A prospective study was carried out of 176 patients (147 males, 29 females; mean age 42.9 years) diagnosed as alcohol dependent according to the DSM-IV criteria in three alcoholic treatment centres in Castilla y León, Spain. 36.9% of the alcohol-dependent patients had had some kind of traffic problem during their life and 8.5% in the year prior to starting treatment. The most frequent problem was positive breath tests, followed by accidents with damage to the vehicle. Sixty-nine of the 176 patients were still receiving treatment after a year. The prevalence of traffic problems among those patients who followed treatment for 1 year (4.3%) was lower than in the year before treatment (15.9%). The study showed that the treatment is also effective in reducing traffic problems.