Serum lysozyme levels were determined in healthy volunteers, patients with Crohn's disease, and patients with ulcerative colitis. The mean concentration in Crohn's disease was significantly greater than in the other groups. In patients with Crohn's disease, as well as in patients with ulcerative colitis, the lysozyme levels correlated with the severity of the disease process and with the extent of the lesions: the more severe the disease and the more extensive the involvement, the higher the lysozyme levels. However, the lysozyme values of the different groups overlapped considerably. Our results indicate that lysozyme determinations have only limited discriminative value for the diagnosis of Crohn's disease and for determining the severity and the extent of the disease process in the individual patient.