We performed this prospective study to evaluate the correlation of interleukin-6 serum levels with preoperative constitutional symptoms and immunologic abnormalities, and the possible role played by this cytokine in tumor recurrence. Eight patients with atrial myxoma were evaluated at our institution from July 1993 to November 1998. We measured their interleukin-6 serum levels by enzyme-linked immunosorbent assay method preoperatively and 1 and 6 months after surgery. Two of the cases involved recurrent tumor, 1 patient had undergone his 1st surgery at a different institution and died during the 2nd procedure, so his data were incomplete. Preoperatively the whole group of patients had elevated interleukin-6 serum levels. Although patients with a 1st occurrence of tumor demonstrated a positive correlation between interleukin-6 serum level and tumor size, the 2 patients with recurrent tumors appeared to have higher interleukin-6 levels regardless of tumor size. Once the tumor was surgically removed, interleukin-6 levels returned to normal values, and this was associated with regression of clinical manifestations and immunologic features. According to our study, the overproduction of interleukin-6 by cardiac myxomas is responsible for the constitutional symptoms and immunologic abnormalities observed in patients with such tumors and might also play a role as a marker of recurrence. This study also suggests that recurrent cardiac myxomas form a subgroup of cardiac myxomas with a highly intrinsic aggressiveness, as implied by their greater interleukin-6 production despite their smaller size. Further studies are needed to confirm these results.