Heme oxygenase (HO)-1, a heme-degrading enzyme inducible by various stimuli, plays a key role in the regulation of inflammatory response in monocytes/macrophages. The serum HO-1 level is remarkably increased in patients with secondary hemophagocytic syndrome (HPS) or adult-onset Still's disease. We measured serum HO-1 levels in patients with a variety of hematological diseases, including secondary HPS, by means of ELISA. Serum HO-1 levels were significantly higher in 22 patients with HPS (134.7 +/- 116.2 ng/mL, P < 0.0001) at diagnosis than in 80 patients with other hematological diseases. The most effective cutoff point between HPS and other conditions was 14.5 ng/mL, with 100.0% sensitivity and 96.3% specificity. In HPS patients, the serum HO-1 levels showed the highest correlation with serum ferritin (r = 0.682, P = 0.0005), which reflects the disease activity of HPS. Moreover, both HO-1 and ferritin levels were reduced in parallel after successful treatment in patients with HPS, irrespective of underlying diseases. However, HO-1 levels were not elevated in patients with other causes of hyperferritinemia. These data demonstrate that serum HO-1 can distinguish secondary HPS from other hematological diseases, including those associated with hyperferritinemia.