The end product of serotonin metabolism, 5-hydroxyindoleacetic acid (5-HIAA), is reduced in the cerebrospinal fluid of both manic and depressed patients. The decrement in this substance, however, does not correlate with severity of illness in either condition. The decrement persists after clinical recovery in depressed patients. These findings must be integrated with current concepts about the role of brain biogenic amines in affective disorders.