The need for a practical screening test to predict whether or not an individual patient will benefit from lithium carbonate therapy grows more urgent with each claim for lithium's use. Serry has suggested that the “Lithium Excretion Test” (LET) (measurement of the amount of lithium excreted in the urine over the 4 hours after a 1200-mg loading dose of lithium) answers this need; he reported that patients who are “lithium retainers” responded to lithium therapy and that “lithium excretors” did not. In a study of a mixed group of 52 subjects, we found that lithium excretion during the LET did not, predict subsequent response to lithium therapy. The LET is therefore useless as a screening test since it would exclude many patients who would benefit from lithium therapy. Our results also differed markedly from Serry's in that lithium excretion in this population fell into a normal distribution with very few “retainers” according to his criteria. It is suggested that the high percentage of “retainers” which he found may be an artifact due to delayed absorption of the lithium carbonate preparation which he administered. The rate of absorption will need to be monitored and controlled in future studies of short-term lithium balance.