This study tested the hypothesis that the level of family functioning is related to changes in the patient's progress while in treatment. We studied the relation of family functioning, as measured by the Family Adaptability and Cohesion Scales' (FACES II) dimensions of cohesion and adaptability, to 7 dimensions of the severity of patient drug use, as measured by the Addiction Severity index (ASI). The results indicated that family functioning, the cohesion dimension in particular, predicts severity of patient's dysfunction resulting from drug use and family and psychological problems. In particular, 28% of the variability in the ASI Drug Problems, 52% of the variability in the ASI Family Problems, and 29% of the variability in the ASI Psychological Problems were accounted for by the FACES II cohesion score. Although the study has several limitations, it supports the relevance of family factors in the treatment of drug-addicted patients.