BACKGROUND. The Global Assessment of Functioning (GAF) is a quick and simple measure of overall psychological disturbance. However, there is little research on the reliability and validity of this measure in severely mentally ill populations. METHOD. Multidisciplinary keyworkers assessed 103 patients at monthly intervals over a 6-month period. Overall GAF scores were obtained, with additional separate ratings for symptoms and disability. These were compared with changes in antipsychotic medication and support needs over the same period. RESULTS. Satisfactory reliability was obtained for total GAF score and for symptom and disability measures, in spite of raters having only one brief training session. All GAF scores were associated with current support needs of patients. Symptom and disability scores were associated with changes in antipsychotic medication in the previous month. Only symptom score was associated with increases in antipsychotic medication at time of rating. CONCLUSION. GAF proved to be a reliable and, within the limits of the indicators used, a valid measure of psychiatric disturbance in our sample of the severely mentally ill. Differences in relationships between the three GAF scores and medication/support needs indicate the usefulness of obtaining all three scores for monitoring levels and type of psychiatric disturbance in this population.