Phagocytes play an essential role in the host's defence against uropathogenic bacteria which are mostly extracellular pathogens. The functional capacity of peripheral blood phagocytes (predominantly polymorphonuclears, PMN) was investigated in 47 patients (urethrocystitis, acute or chronic pyelonephritis) and in healthy persons. The random mobility of phagocytes was determined by measuring their spontaneous migration from a capillary tube. Using radioactively labelled sheep erythrocytes as targets and the phagocytes as effector cells, ingestion, digestion and extracellular cytotoxicity were determined. All the four phagocytic functions in patients were significantly lower than in healthy controls, especially in patients with chronic pyelonephritis. These results link reduced phagocytosis by blood phagocytes with recurrent urinary tract infections (UTI). Whether the defect is primary or secondary to infection (and only transient) should be the object of further studies.