There are no standardized tools in Lithuanian feasible for ongoing routine use to measure the effectiveness of psychotherapeutic treatment. The CORE-OM is a widely used 34-item self-report measure for such purpose.To explore the reliability, validity and sensitivity of the Lithuanian translation of CORE-OM questionnaire.A validation study of the CORE-OM was conducted in the psychiatric clinic attached to Vilnius University. A Lithuanian translation of the English original CORE-OM was prepared by a team of translators. Then 39 psychotherapy outpatients and 187 students were asked to complete the Lithuanian version of the CORE-OM; 66 were tested twice to determine test-retest stability. Analysis included both internal and test-retest reliability, acceptability, influence of gender, principal component analysis and criteria for reliable and clinically significant change.Internal and test-retest reliability were good (0.61-0.94), though somewhat lower for the risk domain (α: 0.57-0.79, Spearman's rho 0.25-0.60). Differences between scores of the clinical and non-clinical samples were large and significant (P < 0.001). Some of the Lithuanian criteria for clinically significant change were a bit lower than those of the original UK criteria (e.g. well-being) and others higher (symptoms, functioning, overall score), illustrating the need for local exploration.In spite of small differences in psychometric properties from the original, the Lithuanian version of the CORE-OM was reliable and sensitive in both clinical and non-clinical settings. It has the potential to become a practical, sensitive and reliable tool for psychotherapists in Lithuania.