A new computer-assisted method for predicting lithium levels and consulting dosage of lithium is demonstrated. The method is based on a mathematical model describing elimination of lithium after lithium treatment of any duration and regularity. From two values of serum concentration of lithium obtained during a single day, parameters of the model are computed and used for prediction. In an evaluation study involving 20 inpatients, the results demonstrated a high correlation between predicted and observed levels (r = 0.80) and revealed no systematic error of prediction. The elimination rate of lithium in these patients was unrelated to age or to duration of previous lithium therapy.