An acute lethal myopericarditis has been observed in four out of fifteen patients receiving high-dose combination chemotherapy which includes cyclophosphamide 45 mg/kg/day for four days. In all cases the myopericarditis occurred 5-9 days after the initiation of chemotherapy, with dyspnoea, tachycardia, orthostatic hypotension, fluid retention, decreased voltage on electrocardiography, and pericardial effusion documented by echocardiogram, and progressed in 2 to 6 days to a fatal low-output state despite vigorous treatment. In three of the four patients, necropsy was permitted and revealed the unique pathological finding of fibrin microthrombi in capillaries, fibrin strands in the interstitium, and fibrin strands within the heart-muscle cells.