Evidence for the existence of categorically distinct disorders such as schizophrenia, bipolar disorder, and major depression is mixed: neuropsychological impairments may be similar in schizophrenia and bipolar disorder; schizophrenia and major depression show similar neuropsychological and frontal lobe disturbances; and overlap in biochemical anomalies among the disorders has also been reported. Interestingly, there are very few studies that directly compare all diagnoses. The present study compares cognitive perseveration in these three diagnostic groups using the Wisconsin Card Sorting Task (WCST) to examine performance across patients with schizophrenia (n =143), bipolar disorder (n =25) and major depression (n =21). Individuals used in this sample were 18–45years old at time of testing to eliminate confounds of aging. Sex ratios within each diagnostic group are comparable to those of the national population. Univariate analyses examining diagnostic group and percent perseverative error revealed no significant differences in WCST performance across the diagnostic groups. Examination of clinical variables in the sample of individuals with schizophrenia revealed that perseveration is related to negative symptoms and depressive symptoms in young adults.