The aim of this study was to examine the psychometric properties of the Swedish version of the Clinical Outcomes in Routine Evaluation–Outcome Measure (CORE-OM) using classical test theory and item response theory (IRT). The CORE-OM is a commonly used 34-item self-report instrument measuring psychological problems/distress covering four domains: subjective well-being, problems/symptoms, functioning and risk. Despite its broad application, only a few studies have used IRT to examine the psychometric properties, and the properties of the Swedish version have only been examined in one initial study. The present study included 1,011 clients with mild to moderate symptoms of distress, applying for psychotherapy at an outpatient training clinic in Sweden. Clients' responses were subjected to classical item analyses as well as IRT (Rasch) analysis using the partial credit model. The classical analyses demonstrated high levels of internal consistency and acceptable levels of item discrimination for the majority of the items, although lower for some items, particularly in the Risk domain. IRT analyses showed that there was a rather good match between item and respondent locations and the measurement precision was high. Disordered step and average measures for some of the items in the Risk domain indicate that these items were problematic from a psychometric point of view and only applicable for a minority of the participants. Differential item functioning for gender in some of the items suggests that they might need to be revised to minimise potential gender bias.