The aim of this study was to estimate the long-term effectiveness of olanzapine as adjunctive therapy in patients with bipolar disorder who exhibited an inadequate response to mood stabilizers. Twenty-three Research Diagnostic Criteria (RDC) patients with bipolar I and II were assessed by means of the Schedule for Affective Disorders and Schizophrenia and entered if they gave their consent to participate. All of them had experienced frequent relapses, residual subsyndromal symptoms, and inadequate responses to other drugs, such as lithium, valproate, or carbamazepine. While maintaining other drugs, they all received open-label, increasing doses of olanzapine, until achieving clinical response. Other drugs were maintained. The patients were assessed several consecutive times from baseline to the endpoint with the Clinical Global Impressions (CGI) scale for use in bipolar illness. Records of recurrences, hospitalizations, and side effects were also collected. The last-observation-carried-forward analysis showed that there was a significant reduction of CGI scores after the introduction of olanzapine, either in manic symptoms (p = 0.0015), depressive symptoms (p = 0.0063), or global symptoms (p = 0.0003). The most frequent adverse events were somnolence (17%) and weight gain (13%). The mean dose of olanzapine at the end of the 43-week follow-up was 8.1 mg/day. Olanzapine may be a useful medication for the long-term adjunctive treatment of patients with bipolar disorder who exhibit a poor response to mood stabilizers, such as lithium, valproate, or carbamazepine. These results suggest mood-stablizing properties of olanzapine.