Suicidal behavior is a major health risk in psychiatric disorders, especially in schizophrenia, wherein up to 10% patients will commit suicide. However, the neurobiology of suicide is still unclear. Suicidality has been related to decreased central serotonergic (5-hydroxytryptamine, 5-HT) function and reduced cholesterol levels. Platelet 5-HT has been used as a peripheral marker of the central serotonergic synaptosomes. The aim of this study was to evaluate serum cholesterol and platelet 5-HT concentrations in suicidal and non-suicidal men in the first episode of psychosis and in healthy male controls. Venous blood samples were collected within 24 h of admission, and serum cholesterol and platelet 5-HT were determined enzymatically and fluorimetrically. Platelet 5-HT and serum cholesterol concentrations were significantly lower in suicidal than in non-suicidal patients in the first episode of psychosis, and also lower than in healthy controls. Our results suggest that lower concentrations of serum cholesterol and platelet 5-HT in patients with a first episode of psychosis might be useful biological markers of suicidality.