Objective: Lithium is recognized worldwide as an effective prophylactic agent in mood disorders. Prophylactic efficacy of lithium in mood disorders has been established since the early seventies. Lithium has been and continues to be the mainstay of bipolar disorder (BD) pharmacotherapy for acute mood episodes, switch prevention, and suicide prevention. There are reports of seasonal variation in lithium levels from a few countries. Variability in the lithium level can lead to a lack of efficacy or to toxicity, making seasonal variation clinically relevant. We aimed to compare lithium levels of bipolar patients between summer and winter.