Objective: This study assessed weight changes in a large cohort of patients with bipolar disorder who were treated with randomly assigned maintenance monotherapies. Method: A post hoc analysis was conducted to assess the effects of lamotrigine, lithium, and placebo administration on body weight in obese and nonobese patients with bipolar disorder from two double-blind, placebo-controlled, 18-month studies. Results: Mean changes in weight among obese patients (N=155) at week 52 were –4.2, +6.1, and –0.6 kg with lamotrigine, lithium, and placebo, respectively (lamotrigine versus lithium and lithium versus placebo). Among nonobese patients (N=399), mean changes in weight (kg) at week 52 were –0.5, +1.1, and +0.7 with lamotrigine, lithium, and placebo, respectively, with no significant differences among groups. Conclusions: Obese patients with bipolar I disorder lost weight while taking lamotrigine and gained weight while taking lithium.