This study evaluates polymorphonuclear neutrophil (PMN) cell performance in 61 diabetic patients free of infection (40 Type 1, 21 Type 2), using tests that explore all the functional steps of PMN: (1) adherence: expression of adhesion molecules, CD 11a, CD 11b, CD 11c; nylon fiber adherence test; (2) chemotaxis under agarose towards the bacterial oligopeptide FMLP and complement fractions, used as attracting agents; (3) phagocytosis of opsonized latex microbeads; (4) bactericidal activity: chemiluminescence assessment of the oxidative killing potential before and after stimulation by opsonized zymosan and PMA; nitroblue tetrazolium reduction test. Results were analysed according to potentially influential factors: metabolic control (HbA 1C , glycaemia), age of patient, type of diabetes, disease duration, and existence of vascular complications. PMN chemotaxis was significantly lower in patients than in healthy controls (p Ο½ 0.001) and associated with spontaneous adherence and increased expression of adhesion molecules (CD 11b, CD 11c). The increased response to chemiluminescence reflects spontaneous activation of PMN cells and increased free radical production; after stimulation, response was lower than in controls. The type of diabetes, the age of patients, HbA 1C level and disease duration did not affect the responses. Chemotaxis and chemiluminescence were further reduced in patients with vascular complications and hyperglycaemia. We conclude that all steps of PMN functioning are altered in diabetic patients, which may increase the risk of vascular complications and infectious episodes. KEY WORDS Polymorphonuclear neutrophils Vascular complications Hyperglycaemia Diabetes mellitus exist in the function of both resting and stimulated Methods Correspondence to: Docteur Maryvonne Delamaire, Centre Re Β΄gional Specimens for PMN study were collected in heparin. de Transfusion Sanguine, rue Pierre-Jean Gineste, 35000 Rennes, France The following steps of PMN functioning were studied: Abbreviations: FMLP formyl-methionyl leucyl phenylalanine, MLI adherence to nylon fibres, intensity of adhesion molecule maximal light intensity, NBT nitroblue tetrazolium, PMA phorbol myristate acetate, PMN polymorphonuclear neutrophils expression, chemotaxis, phagocytosis and killing (using 29