Increments in serum ferritin levels in adult onset Still's disease (AOSD) were reported to be higher than one could expect for a simple inflammatory state. When we analyzed the scores of 40 patients with various severe inflammatory diseases aside from AOSD, we recorded no serum ferritin values higher than 3,300 ng/ml (N less than 200 ng/ml). In 3 of 10 consecutive patients with AOSD, the ferritin levels were higher than 3,500. Among these 3 patients, one case had a ferritin value of 3,600 ng/ml and bone marrow aspirate showed a marked hyperplasia of mature appearing histiocytes, and the 2 other patients (serum ferritin levels of 65,000 ng/ml and 250,000 ng/ml) displayed the features of a hemophagocytic syndrome. In 2 patients with normal or mildly increased levels of ferritin, the bone marrow examination was normal. We suggest that very high serum ferritin levels encountered in AOSD reflect the presence of histiocytic hyperactivity that sometimes leads to a hemophagocytic syndrome.