Background: We evaluated the antidepressant and mood-stabilizing effects of lamotrigine, a novel anticonvulsant, in a group of rapid-cycling bipolar patients. Most were already nonresponders or poor partial responders to other conventional mood-stabilizing agents. Methods: This open, naturalistic, and prospective study was conducted with five rapid-cycling bipolar patients (DSM-IV). Each received lamotrigine titrated to a minimum dose of 150 mg/day as monotherapy or in combination with other psychotropic agents. Patients were assessed with the Global Assessment Scale (GAS), Beck Depression Inventory (BDI), and Young Mania Rating Scale (YMRS) for evidence of cycling mood. Results: Lamotrigine was used at a mean ± SD dose of 185.0 ± 33.5 mg/day for 225.8 ± 28.0 days. Random regression modeling of data showed significant dose- and time-dependent improvements in depressive symptoms and social function of patients taking lamotrigine (Dose: z = 2.17, p < .03 for BDI, z = 4.44, p < .001 for GAS; Time: z = -3.79, p < .001 for BDI, z = 2.16, p < .03 for GAS). Further random regression modeling analysis of change over time in symptoms prior to lamotrigine compared with symptoms during lamotrigine treatment showed a significant treatment by time effect for GAS (z = 2.40, p < .016) and a trend for BDI scores (z = -1.79, p < .073). No significant time or dosage effect or time by treatment effect was observed for YMRS. Finally, t statistics showed a significant reduction in mean BDI scores following treatment with lamotrigine (t = -5.26, p < .006). Lamotrigine was well tolerated by all patients; only one patient experienced several side effects, which were probably due to interaction between several psychotropic medications. Conclusions: Lamotrigine augmentation therapy and monotherapy appeared to have mood-stabilizing and antidepressant efficacy in the treatment of five rapid-cycling bipolar patients. The effect persisted for an average of 7.5 months.