Summary:The utility of the Wisconsin Card Sorting Test (WCST) to effectively differentiate diagnostic groups was evaluated. Performance on the WCST was contrasted among samples of schizophrenic, mood-disordered, and traumatically brain-injured patients, as well as normal controls. The controls performed significantly better than all three patient groups, without any differentiation among the patient samples. The pattern of performance, over time, also did not differ between the groups. The poor discrimination between the patient groups may provide evidence for multiple cognitive processes being involved in WCST performance (e.g., attention, executive functioning). Consequently, similar WCST performance could result from deficiencies in different cognitive processes.