Peripheral blood specimens from 22 hospitalized psychiatric patients and one outpatient were examined for white blood cell (WBG) count changes during treatment with lithium carbonate. Significant leukocytosis occurred during periods of lithium ingestion; this phenomenon was reversible, apparently innocuous, and not related to psychiatric diagnosis or the many variables of hospitalization. While elevations in WBG count are likely the result of a drug effect, they were not dose related or dependent on the concentration of lithium in the peripheral blood. A trend toward neutrophilia and lymphocytopenia emerged for the group but cannot be said to account for the global change in total WBG.