We report the case of a 40-year-old man with Klinefelter's syndrome who presented with a complex multisystem illness, the predominant manifestations of which were rheumatological and cardiological. This disorder coincided with a diagnosis of acute myelogenous leukemia evolving out of a myelodysplastic syndrome and resolved completely after chemotherapy. It is concluded that the hematological malignancy probably played a causal role in the development of this unusual illness which was likely immunologically mediated.