Proliferating cell nuclear antigen (PCNA) is a 36-kD nuclear protein that functions as a cofactor of delta DNA polymerase which is regulated in a cell cycle-dependent fashion. PCNA expression also increases when cells are actively engaged in DNA repair. We used Western blotting (WB) to measure the level of expression of PCNA in peripheral blasts of 36 adult acute myelogenous leukemia (AML) patients treated with Ara-C based induction regimens. PCNA levels correlated positively with the percentage of cells in S+G2M of the cell cycle. Logistic regression analysis revealed PCNA (beta = 4.5162; p = 0.0260) together with age (beta = 0.1777; p = 0.0364) as independent variables for remission induction: high PCNA levels were associated with poor response to induction therapy. PCNA expression was not, however, a predictor of survival in this subset of patients. We conclude that PCNA levels in this disease may be important for predicting response to Ara-C based remission induction chemotherapy.