Clinical features in patients with haemophagocytic lymphohistiocytosis (HLH) have been demonstrated to be characterized by hypercytokinaemia. Previously, we reported the impact of high serum levels of interferon (IFN)-gamma and soluble IL-2 receptor (sIL-2R) on patient outcome; however, it was not known if serum levels of interleukin (IL)-6 also could be a prognostic factor. In a study during the active phase of disease in 25 cases of HLH in children and young adults (median age 3 years, range 0.1-23 years), we noted 12 cases which showed serum IL-6 > 100 (normal < 4.0) pg/ml. Five of these cases showed hyper-IL-6-naemia alone without hyper-IFN-gamma-naemia (group A) whereas seven cases showed both hyper-IL-6- and IFN-gamma-naemia (group B). Patient outcome did not differ between the patients with IL-6 > 100 pg/ml and those with IL-6 < 100 pg/ml, suggesting that high serum concentrations of IL-6 alone do not necessarily indicate poor prognosis in patients with HLH. Among the cases with hyper-IL-6-naemia ( > 100 pg/ml), underlying disorders causing haemophagocytosis were found to be different between groups A and B.