A literature review was conducted to locate studies that compared different serum lithium levels in the long-term treatment of patients with bipolar disorder and articles about factors that may affect serum lithium levels. Patients with bipolar disorder on long-term treatment with lithium are typically maintained at serum lithium concentrations between 0.6 and 1.0 mEq/L. Although there are individual exceptions, serum lithium levels below 0.6 mEq/L have been shown in controlled clinical trials to be less effective in preventing relapses than levels within this range, whereas levels much above 1.2 mEq/L can lead to toxicity. Differences in efficacy between levels within the accepted range have not been established. However, higher levels are associated with greater side effects, which can lead to poor compliance. Interindividual variation in pharmacokinetics and pharmacodynamics, as well as such external factors as diet and concomitant medications, can affect serum lithium levels.