Several lines of research suggested that aripiprazole might be a useful treatment for acute bipolar depression. The aim of this open-label trial is to give more evidence of the clinical effectiveness and tolerability of aripiprazole in acute bipolar depression. Aripiprazole response was prospectively assessed for 16 weeks using the Montgomery-Asberg Depression Rating Scale (MADRS), the Clinical Global Impression Severity Scale (CGI-S), and the Young Mania Rating Scale in 85 bipolar patients with acute depression inadequately responsive to one mood stabilizer. Aripiprazole was well tolerated. Only three (3.5%) patients discontinued the study for side effects. The most common side effect was akathisia, occurring in 17/80 (21.2%) patients. Patients showed statistically insignificant weight gain (0.9 +/- 2.64 kg) over the 16-week trial. Patients showed a significant decrease in mean MADRS and CGI-S, and 80 (94.1%) patients completed the 16-week trial. Thirty-nine (45.8%) patients received aripiprazole as monotherapy and 46 received the drug adjunctively (54.1%). Fifty-two (65%) patients met criteria for response (>/= 50% reduction in MADRS total score), 30 (37.5%) patients met criteria for remission (final MADRS total score