Chinese herbal medicines possess the therapeutic potential for mood disorders. This double-blind, randomized, placebo-controlled study was designed to evaluate the efficacy and side effects of the herbal medicine called Free and Easy Wanderer Plus (FEWP) as an adjunct to carbamazepine (CBZ) in patients with bipolar disorders. One hundred and twenty-four bipolar depressed and 111 manic patients were randomized to treatment with CBZ alone, CBZ plus FEWP, or equivalent placebo for 12 weeks. CBZ was initiated at 300mg/day and FEWP was given at a fixed dose of 36g/day. Efficacy measures included the Hamilton Rating Scale for Depression, Montgomery-Asberg Depression Rating Scale , Young Mania Rating Scale, Bech-Rafaelsen Mania Scale, and Clinical Global Impression-Severity (CGI-S). CBZ monotherapy produced significantly greater improvement on manic measures at week 2 through endpoint and CGI-S of depression at endpoint compared to placebo. CBZ monotherapy also yielded significantly higher clinical response rates than placebo on bipolar depression (63.8% vs. 34.8%, p=0.044) and mania (87.8% vs. 57.1%, p=0.012). Compared to CBZ monotherapy, adjunctive FEWP with CBZ resulted in significantly better outcomes on the three measures of depression at week 4 and week 8 and significantly greater clinical response rate in depressed subjects (84.8% vs. 63.8%, p=0.032), but failed to produce significantly greater improvement on manic measures and the response rate in manic subjects. There was a lesser incidence of dizziness and fatigue in the combination therapy compared to CBZ monotherapy. These results suggest that adjunctive FEWP has additive beneficial effects in bipolar patients, particularly for those in depressive phase.