An increasingly important consideration in drug abuse policy and programming is the growing number of multiple substance abusers, i.e., problem-drinking drug addicts. A longitudinal study of two drug addict populations examined drug and alcohol usage, psychological variables, and criminal justice and employment indicators. Findings indicated that alcohol abuse was a highly meaningful and clinically useful indicator of pervasive problems and special treatment needs of drug clients. Poor treatment outcome was most strongly associated with problem drinking at the time of follow-up. No increase in problem drinking existed among methadone clients. The strongest rehabilitative effects exhibited after a year of methadone maintenance included control of narcotic abuse and improved employment records. (JAC)