Suicidal behavior is a major health risk in psychiatric disorders, especially in schizophrenia, and up to 10% patients will commit suicide. The neurobiology of suicide is still unclear. Suicidality has been related to a decreased central serotonergic (5-hydroxytryptamine, 5-HT) function and reduced cholesterol levels. Platelet 5-HT was used as a peripheral marker of the central serotonergic synaptosomes. The hypothesis was that suicidal patients in the first episode of psychosis will have different serum cholesterol and platelet 5-HT concentrations than non-suicidal patients in the first episode of psychosis. 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 hours 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 than in healthy controls. Our results suggest that lower concentrations of serum cholesterol and platelet 5-HT in patients with the first episode of psychosis might be useful biological markers of suicidality.