The Research Diagnostic Criteria (RDC) define endogenous depression by the presence of a particular subset of depressive symptoms. This typological approach to classification implicitly assumes that endogenous and non-endogenous patients differ only or primarily in this subset of symptoms, rather than simply in severity of all depressive symptoms. We tested this assumption in a sample of 80 patients with a current episode of major depressive disorder. Whereas RDC endogenous patients (n = 53) had significantly higher levels of most criterial symptoms than did non-endogenous patients (n = 27), they differed on almost no non-endogenous symptoms. These findings support the concept of a specific endogenous symptom cluster.