Physical findings, laboratory data, treatments and prognosis were investigated in detail using 26 Japanese childhood Still's disease (CHSD) patients and 19 Japanese adult onset Still's disease (AOSD) patients as the subjects. High spiking fever and arthritis were present in all the patients. Seventy and seven percent of CHSD and 53 percent of AOSD had polyarthritis (the number of joints involved being 5 or more during the first 6 months of the disease). A comparison of the groups showed no significant difference in the initial systemic manifestations except for sore throat (CHSD: AOSD; 19%: 68%). Initial laboratory data were the same for these groups except for serum iron levels (CHSD: AOSD; 20.8 +/- 13.7 micrograms/dl: 83.0 +/- 54.2 micrograms/dl). As to joints and physical prognosis, the results were also the same for CHSD and AOSD under the similar treatment. On the basis of these data, we conclude that CHSD and AOSD are of the same disease entity so far as the present clinical features are concerned.