PurposeAlcohol use problems are prevalent among recidivists of driving under the influence of alcohol (DUI). This 2-year postintervention follow-up study explored the preventive effects of 12- or 6-month alcohol treatment in relation to reoffense among repeat DUI offenders who participated in a community-based joint legal–medical intervention program. MethodsA total of 259 repeat DUI offenders with alcohol use problems were referred from a Prosecutors’ Office to a psychiatric hospital to receive alcohol treatment. We divided these participants into two groups on the basis of treatment duration (6- and 12-month groups) and collected their official DUI offense records for 2 years after the intervention. A Cox proportional hazards model was used to examine the hazard ratio of each treatment duration in relation to reoffense. The reoffense rates were compared with a group of DUI offenders (n = 4097) who did not participate in the intervention program (i.e., a nonintervention group). ResultsThe baseline characteristics were similar between the two treatment groups. The 2-year reoffense rates for the 12-month treatment, 6-month treatment, and nonintervention groups were 12%, 19%, and 21%, respectively. The hazard ratio for reoffense was lower for the 12-month group than for the 6-month group (hazard ratio: 0.45; 95% confidence interval: 0.21–0.96). Survival analysis revealed that compared with the non-intervention group, reoffense was significantly reduced in the 12-month group but not in the 6-month group. ConclusionThe 12-month joint legal–medical intervention program for alcohol treatment yielded superior outcomes to the corresponding 6-month program in terms of preventing DUI reoffense for 2 years postintervention. This research is, however, subject to the limitation that we lacked the information of factors that might affect the reoffense risk, such as motivation level, readiness to change, personality traits, and familial support.