Urinary levels of lysozyme activity were measured in 32 children afflicted with acute or chronic urinary tract infection and compared to a group of 30 healthy subjects. A significant difference in the levels of lysozyme activity between the two groups could be observed. In the subjects with acute urinary tract infection the lysozyme levels in the urine were additionally determined at the beginning of the therapy, on the third day, the tenth day and three days after cessation of therapy. High lysozyme levels encountered at the onset of the infection showed under therapy a clear tendency to decrease and in all cases no lysozyme was present three days after therapy was completed. The possible causes of the pathological lysozymuria are discussed. The determination of lysozyme is thus an additional method by which to control the course and outcome of urinary tract infections.