Serious suicidal behavior, affective disorders, and a variety of other psychopathologic behaviors and syndromes have been found to correlate with measures of the serotonin system. Clinical studies have employed a range of serotonin indexes, including the cerebrospinal fluid level of 5-hydroxyindoleacetic acid, the prolactin response to serotonin agonists, such as fenfluramine hydrochloride, and platelet serotonin-related proteins or serotonin content. Many of these indexes are correlated with suicidal behavior, but the interrelationship of these biologic measures has been uncertain. We studied the relationship of a series of serotonin indexes in patients in whom these measures were correlated with suicidal behavior. A positive correlation was found between cerebrospinal fluid 5-hydroxyindoleacetic acid and the maximal prolactin response to fenfluramine but not with platelet serotonin2 receptor indexes. The fenfluramine-stimulated maximal prolactin response correlated with platelet serotonin2 receptor number, particularly in older patients. We conclude that cerebrospinal fluid 5-hydroxyindoleacetic acid measurements cannot be replaced but can be complemented by less invasive procedures, such as a fenfluramine challenge test or platelet serotonin2 measures, in the study of the relationship of the serotonin system to psychiatric disorders.