A 17-year-old male patient with aplastic anemia underwent bone marrow transplantation and succumbed 4 days after marrow infusion from sudden myocardial failure. Fever of unknown origin (FUO) had accompanied the patients course from admission until death. The cause of death was fungus myocarditis, which had escaped detection in vivo, in spite of a daily culture program for bacteria and fungi, and a close monitoring of the patients circulation and ventricular performance. Commonly applied diagnostic criteria for systemic mycosis, such as topical colonization, malfunction of invaded organs and positive fungus cultures failed to provide a timely diagnosis. With regard to the problems in diagnosing systemic mycosis, the potential stem cell toxicity of antifungal drugs and the need for immunosuppressive therapy prior to marrow infusion, we strongly recommend not to start the transplantation procedure unless FUO has been treated successfully.