The entry for nonsuicidal self-injury (NSI) disorder in the 5th edition of the Diagnostic and Statistical Manual o f Mental Disorders provides a criterion-based definition of clinically relevant NSI. NSI disorder is currently classified in the DSM-5 as a condition requiring further study. The present study aimed to examine the reliability, validity, and clinical utility of a self-report measure of NSI disorder, the Alexian Brothers Assessment of Self-Injury (ABASI). The sample included 511 patients admitted to an acute care treatment program designed to treat NSI. Patients were administered the ABASI as part of a clinical assessment and routine outcome evaluation. The sample included a broad age range, as well as sufficient numbers of males and Hispanics to examine sociodemographic differences. The ABASI demonstrated adequate internal consistency and test-retest reliability, and the factor structure reflects NSI disorder criteria. Among patients being treated for NSI, 74% met criteria for NSI disorder. No differences in the rate of NSI disorder were observed by sex, ethnicity, or age. Although NSI disorder is associated with a worse presentation of self-injurious behavior, NSI disorder provides limited clinical utility as a dichotomous diagnosis, at least when compared with common NSI characteristics such as number of methods of NSI and the urge to self-injure. Instead, findings support a dimensional approach to NSI disorder. Analyses of specific symptoms of NSI disorder indicate concerns with Criterion B as currently defined by the DSM-5. Recommendations for a more parsimonious revision of NSI disorder are discussed.