The purpose of this study was to conduct the first prospective examination of the utility of the tryptophan ratio along with clinical variables in predicting suicidal behavior in high-risk & reference adolescents. Adolescents with alcohol use disorders (AUD) & prior suicidal attempts, those with AUDs & without suicide attempts, & community controls with neither of these characteristics were compared. The tryptophan ratio & major depressive disorder both significantly contributed to the prediction of suicidal behavior. (32 refs)