Severe infections are a major problem in patients suffering from acute nonlymphocytic leukemia (ANLL) undergoing myeloablative chemotherapy. Possible factors leading to infectious complications in these patients are suppressed immune defense mechanisms existing prior to therapy, including those involving the neutrophil granulocyte department. In this study we investigated whether neutrophil function as measured by oxidative burst and phagocytosis before the start of treatment correlates with the severity of infection after therapy. Forty-four patients were included, 27 men and 17 women. Their median age was 46 years (range 20-70 years). According to the development of infectious complications the patients were assigned retrospectively to group 1 (no or only mild infections, n = 29) or to group 2 (severe infection or death due to infection, n = 15). The phagocytic activity was significantly reduced in group 2 as compared with group 1 [113.7+/-13.7 (SEM) vs 170.0+/-19.2, mean channel fluorescence; p =0.04]. In contrast, the oxidative burst as measured by FMLP stimulation was pronounced but not significantly enhanced in group 2 (24.8+/-6.1 vs 14.5+/-3.4, mean channel fluorescence). In conclusion, patients with severe infections after chemotherapy might already have preactivated neutrophils with suppressed function prior to treatment. Thus, evaluating function parameters could help to estimate the individual risk of infection for a patient with ANLL.