State Psychiatric Hospital outpatients (93 men, 69 women) diagnosed with a serious persistent mental illness (SPMI) completed the Alcohol Use Disorders Identification Test (AUDIT) and Drug Abuse Screening Test (DAST-10) by interview as part of a general health/behavior screening instrument. Responses to the AUDIT and DAST-10 were compared with criteria of current diagnosis and occurrence of symptoms in the last year for both alcohol and drug use disorders, respectively. Results showed that for both diagnosis and symptoms, AUDIT cutpoints of 7 and 8 had good sensitivity and specificity, and DAST-10 cutpoint of 2 was identified for both criteria. These and other findings suggest that both instruments have promising clinical utility when used with individuals diagnosed with an SPMI.