To get more information about the high infection rate in splenectomized adult patients 211 spleenless patients were investigated with regard to clinical and laboratory data and compared to healthy blood donors. The results show that the infection rate is markedly increased to 30%. Splenectomized patients have decreased IgG levels which is due to diminished IgG1 and IgG4. Whereas IgA, complement factors C3, C4, and transferrin are not changed in patients without spleen, fibronectin and IgM are significantly reduced and the phagocytosis as well as the migration of neutrophilic granulocytes is impaired to 50%. With these changes in laboratory data it is possible to identify patients which bear an increased risk with regard to infection.