The concentration of 5-hydroxyindoleacetic acid (5-HIAA) in the cerebrospinal fluid (CSF) and psychophysiologic variables, such as habituation of the skin conductance response, were measured in 35 drug-free, suicidal inpatients. Twenty-four patients were hospitalized after a suicide attempt, and another 11 had suicidal ideation. The suicide attempters were classified into nonviolent (drug overdoses taken orally, or a single wrist cut) and violent (all other methods). As in previous studies, the suicide attempters had significantly lower CSF concentrations of 5-HIAA compared with healthy, matched controls. The distribution of habituation rate was bimodal (slow and fast habituators). All violent attempters were fast habituators, as were all four patients who in a one-year follow-up were found to have completed a suicide (all by violent methods). The lowest frequency of fast habituators was found in the group of patients with suicidal ideation. There was no correlation between CSF 5-HIAA and habituation rate. A combination of these two variables yielded a highly significant correlation with type of suicide behavior, both retrospectively and prospectively.