This study analyzes the psychometric and structural properties of the BSI-18 in a sample of Spanish outpatients with psychiatric disorders (N = 200), with three basic objectives: (a) to study the structural validity of the instrument; (b) to analyse reliability (internal consistency and test-retest stability) and validity (convergent and discriminant) of the instrument, and (c) to evaluate sensitivity to change in a therapeutic context. Using confirmatory factor analysis, two models were tested: the three-dimensional theoretical model proposed by its authors, and the empirical four-factor model obtained by the same authors through exploratory factor analysis. Our results showed that the best structure was a four-dimensional model, with the General Distress index and Somatization, Depression, General Anxiety and Panic scores. These four scales showed good internal consistency, test-retest reliability, validity and sensitivity to therapeutic change. The BSI-18 has been shown to be a reliable and useful tool for psychiatric assessment of patients, with the added advantage provided by its simplicity and ease of application.