The incidence of suicidal acts was studied in 68 depressed patients and related to the level of 5-hydroxyindoleacetic acid (5-HIAA) in the cerebrospinal fluid. The distribution of 5-HIAA levels was bimodal. Patients in the low 5-HIAA mode (below 15 ng/ml) attempted suicide significantly more often than those in the high mode, and they used more violent means. Two of the 20 patients in the low mode, and none of the 48 patients in the high mode died from suicide.