A retrospective study was made of a series of 73 children who presented at the Diagnostic and Developmental Center, Northbrook, IL, U.S.A. between February 1, 1983 and July 1, 1984 with learning disabilities, impulsivity, short attention span, and in some cases, hyperactivity as well. Ninety-eight percent of the patients with low whole blood serotonin levels (below 90 ng/ml) improved in their ability to pay attention and learn and diminished in impulsivity when the psychostimulant medication pemoline was administered, while the patients with higher levels (above 100 ng/ml) did not benefit from the drug. Thus, the measurement of serotonin levels in whole blood appears to have predictive value as to which children can be helped by treatment with pemoline. Based on the data presented, we recommend the use of the whole blood serotonin test at the time of initial evaluation to determine whether children should be treated with pemoline, thereby sparing those who would not respond favorably from an ineffectual trial of the drug.