OBJECTIVE: To determine if there is a relation to low serum cholesterol, lipoprotein, serotonin or tryptophan levels in patients with depression who have recently attempted suicide. DESIGN: Biochemical and behavioural study. SETTING: Inpatient and outpatient treatment at the Instituto Mexicano de Psiquiatría. PARTICIPANTS: Thirty-three patients with a diagnosis of major depressive episode. Eighteen of these patients had attempted suicide in the month before the start of the study; 15 patients had never attempted suicide. OUTCOME MEASURES: Serum levels of total cholesterol, high-density lipoprotein (HDL), low-density lipoprotein (LDL), triglycerides, serotonin (5-HT) and tryptophan. Scores on Hamilton Depression Rating Scale, Carroll Depression Rating Scale, Beck Hopelessness Scale and Beck Suicide Attempt Severity Scale. RESULTS: There were no significant differences between patients who had attempted suicide and those who had not in terms of serum cholesterol, HDL, LDL and triglyceride levels. Serum levels of 5-HT and tryptophan were significantly lower in patients with depression who had a recent suicide attempt than in those patients who had never attempted suicide. A comparison of patients not taking antidepressant medication found serum 5-HT levels to be more than 3 times lower in those patients with a recent suicide attempt than in patients with no history of suicide attempt. CONCLUSIONS: The study found no difference in lipid profiles between patients who had attempted suicide and those who had not. Low serum levels of 5-HT may increase the risk of suicide attempt in patients who are depressed.