Objective: To assess the adequacy of global ratings of patients' psychosocial functioning, which are an integral part of the current system for obtaining multidimensional psychiatric diagnoses and are embodied by the Global Assessment of Functioning (GAF) Scale as AXIS V of the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (American Psychiatric Association, 1994). Method: We identified a sample of 1,688 patients with substance use disorders, many of whom also had psychiatric disorders; examined the determinants of GAF ratings; and focused on how well these ratings predicted patients' one‐year symptom and psychosocial functioning outcomes. Results: Patients' clinical diagnoses and psychiatric symptoms were stronger predictors of GAF ratings than was their social and occupational functioning. Moreover, GAF ratings were only minimally associated with patients' one‐year psychological, social, and occupational functioning outcomes. Conclusions: These findings raise serious questions about the conceptual and clinical value of the current standard method of assessing psychiatric and substance abuse patients' global functioning. © 2000 John Wiley & Sons, Inc. J Clin Psychol 56: 449–461, 2000.