Splenectomy and functional hyposplenism predispose the patient to serious life threatening infections. The most frequent pathogenic organism which causes serious infections in splenectomized adults or hyposplenic patients is Streptococcus pneumoniae. In the etiology of these conditions however also a number of other bacterial and non-bacterial microorganisms may be involved. The risk of serious infections and deaths may be markedly reduced by simple preventive measures, education of the patient, vaccination and antibiotic prophylaxis.