This study reports on a comparative empirical investigation into the effects of treating alcoholics and drug addicts together as opposed to separately. Four hundred fifty subjects who had been randomly assigned to combined and separate treatment were followed eight months post treatment. The study reports on general effects of outcome of treating alcoholics and drug addicts together rather than separately and moves on to ascertain whether certain subgroups of alcoholics and drug addicts might do better or worse in combined or separate treatment. In general, neither drug addicts nor alcoholics did worse on any of the outcome measures. Furthermore none of the subgroups did better or worse in combined or separate treatment. The study provides support for combined treatment of alcoholics and drug addicts as a feasible alternative to separate treatment.