Adaptive functioning/impairment is an important mental health outcome domain that is conceptually distinct from symptom severity. The Global Assessment of Functioning (GAF; American Psychiatric Association, 1994) is the most commonly used measure of adaptive functioning/impairment in mental health settings. We suspect that GAF scores may be influenced by factors other than functional impairment. In this study, three raters classified, with high reliability, the reasons given by eight clinicians for 80 GAF ratings. As expected, GAF ratings were strongly influenced by factors other than adaptive functioning/impairment, like symptom severity. The GAF is not a good measure of adaptive functioning, yet important decisions affecting clinicians and clients are made on the basis of GAF scores. Better measures of adaptive functioning are needed.