Perfobrmance on the Subtraction of Serial Sevens Test (SSST) was evaluated in 325 patients with severe mental disorders and an agematched, nonhospitalized group of siblings of 50 of these patients. Approximately 65% of the patients completed the test and half of these demonstrated errorless performance. Of the remainder, only relatively few individuals made more than two errors. The number of errors made during the SSST was complexly related to the final psychiatric diagnosis as well as to intelligence, but not to age or number of years of schooling. Independent of diagnosis, performance also related to ability to attend, concentrate, and understand. Performance was not associated with clinical or laboratory indicators of possible organic impairment. Likewise, there were no significant differences between the 50 patients and their nonhospitalized sibling controls.