The authors used the family study method to test the hypothesis that sodium lactate response defines two subtypes of panic disorder.Rates of panic disorder in 142 first-degree relatives of patients who responded to sodium lactate, 88 first-degree relatives of patients who did not respond to sodium lactate, and 231 first-degree relatives of never mentally ill subjects were compared.No difference in familial transmission of panic disorder was found between the two patient groups.The findings do not support the notion that panic disorder subtypes are associated with lactate sensitivity.