The prognostic significance of Auer rods in predicting response rate and remission duration was investigated in 257 patients of the two West German acute myelogenous leukemia (AML) studies BFM-78 and -83. Auer rods were found in 129 children (50%) in the initial bone marrow smear. The incidence was higher in myeloid subtypes M1 (63%) and M2 (78%) compared with subtypes M4 (47%) and M5 (5%) with monocytic differentiation. In both studies, the remission rate was significantly higher in patients with Auer rods (P = 0.01), and in the study AML-BFM-83 a significantly longer remission duration was evaluated for Auer rod-positive patients (P = 0.007). In the M1 type, Auer rods were of high prognostic significance regarding both the induction success (P = 0.003) and the remission duration (P = 0.004), whereas no significant differences between Auer rod-positive and -negative patients in other subtypes were found. Occurrence of Auer rods was independent of hyperleukocytosis, the most powerful prognostic parameter in the studies AML-BFM-78 and -83, whereas absence of Auer rods was associated with the AML risk group M5. In the M1 type, Auer rod-negative leukemias appears to represent cases of poorly differentiated AML. Auer rods were therefore useful in differentiating between patients with a poor and a more favorable prognosis, particularly in the M1 type.