Luminol-amplified chemiluminescence was used to study the oxidative metabolism of polymorphonuclear leukocytes (PMN), in resting state and in response to opsonized zymosan, in 65 patients with different degrees of chronic renal failure (CRF) or on regular dialysis treatment (RDT). Every patient was compared on the same day with a normal subject. Furthermore, the serum opsonic activity was evaluated, cross-matching zymosan opsonized by serum from CRF-RDT patients and normals with PMN from CRF-RDT patients and normals. PMN resting chemiluminescence showed a progressive increase inversely related to the glomerular filtration rate, and it remained high in patients on RDT. Zymosan-activated chemiluminescence indicated a deficit in phagocytosis for PMN of patients with a glomerular filtration rate lower than 10 ml/min, persisting in RDT patients. The serum opsonic activity was always significantly lower in CRF and in RDT patients than in the control group; this defect was already present in patients with mild renal impairment. Our findings suggest that PMN from CRF or RDT patients have an increased reactive oxygen metabolite production in the resting state that may cause cell and tissue damage; the opsonization impairment and the decreased PMN phagocytic activity contribute to increased vulnerability to infection in these patients.