A 6-y-old girl with right atrial myxoma presented with remittent fever attacks, general arthralgia and laboratory investigations mimicking rheumatic or autoimmune disease. Interleukin-6 (IL-6) serum concentration was markedly elevated before and normal after tumour resection, whereas myxoma cells stained negatively for IL-6. IL-6 should be considered a myxoma marker: overproduction by myxoma cells and consecutive systemic passage are assumed to cause immunological features.