OBJECTIVE: To determine the effect on the serum lithium levels of a psychoeducational program in patients with bipolar disorder. METHOD: This is a subanalysis of data obtained from a larger study on the efficacy of group psychoeducation in the prophylaxis of recurrences in bipolar disorders. Data on plasma lithium levels were obtained at five time points: baseline, 6, 12, 18 and 24 months. Serum lithium levels of patients who had received psychoeduction (psychoeducated) (N = 49) and non-psychoeducated patients (N = 44) were compared. RESULTS: Mean serum lithium levels were significantly higher and more stable for the psychoeducation group. CONCLUSIONS: As changes in serum lithium level may be a powerful predictor of recurrence for bipolar patients, the addition of group psychoeducation to standard pharmacological treatment may be beneficial to optimize serum lithium levels and, thereby improve outcome.