Potent interleukin‐1 (IL‐1) activity was detected in culture supernatants from synovium, obtained by arthroscopy, from rheumatoid arthritis (RA) patients but not from non‐RA patients. Production of IL‐1 by RA synovium correlated well with findings of inflammation on arthroscopy and HLA–DR expression in immunohistochemical staining. Furthermore, there was a positive correlation between IL‐1 production from RA synovium and joint changes detected on roentgenograms. These findings strongly suggest that IL‐1 might play an important role in the joint destruction in RA.