Cytomegalovirus infections are frequent and often life-threatening in bone marrow transplant recipients. Among 112 of our patients after bone marrow transplantation (BMT), 28 developed clinical symptoms or laboratory test results of CMV infection, and 15 of them died. In our experience blood cell substitution with products from CMV negative blood donors does not completely prevent CMV infections. Other factors like disturbances of immunoreconstitution after BMT are relevant in promoting the development of CMV disease.