SummaryAlthough lithium is a mainstay in the treatment of mood disorders, its use is often accompanied by neuropsychological side effects that may contribute to a high rate of medication noncompliance. These deficits, including subjectively slowed information-processing speed, a sense of lethargy, and memory disturbance, are similar to those seen in patients with hypothyroidism, whether overt or subclinical. This comparison is relevant because lithium is a potent antithyroid drug, and it has been hypothesized that at least some of the neuropsychological deficits caused by lithium are attributable to its antithyroid properties. As a preliminary test of this hypothesis, the neuropsychological functioning of 16 psychiatric outpatients taking lithium, eight of whom were determined to be subclinically hypothyroid and eight of whom were euthyroid, was assessed. Subclinically hypothyroid patients were found to perform significantly worse on measures of verbal learning and memory than their euthyroid counterparts. Performance on these measures was more highly correlated with thyrotropin levels than with serum lithium levels. These preliminary results are consistent with the hypothesis that some deficits are caused by lithium and may have implications for the treatment of lithium's neuropsychological side effects.