If clinically evident synovitis causes erosions, then the two should be highly correlated within individual joints. Separate hand joints (total 2064) were examined for the presence of synovitis (the simultaneous presence of soft-tissue swelling and tenderness) on nine occasions over 2 yr. The cumulative synovitis score was compared to the change in the Larsen score over the same period. The mean correlation between synovitis and erosion progression was r = 0.248 (explained variance = 6%). Of the 216 joints which showed progressive X-ray damage, 44% had a cumulative synovitis score of < 3. When all the joints of one hand were taken together, the correlation was increased to r = 0.418 and for all the joints of both hands taken together it was further increased to r = 0.424. These results argue against there being a direct causal relationship between clinically inflamed synovitis and erosions in rheumatoid arthritis, and question the assumption that erosions and the signs of synovitis represent the same pathological process.