Thirty-seven patients with classical or definite rheumatoid arthritis (RA) were studied prospectively over a period of 12 months to assess whether there was a relationship between disease activity and raised levels of antibodies to native or denatured type II collagen. Nineteen patients had inactive RA according to ARA criteria for disease remission and 18 had active RA throughout the study. At the beginning of the study the levels of collagen antibodies were comparable in each group. After 1 year, antibodies to denatured type II collagen in patients with inactive RA had declined to significantly lower levels whereas in patients with active RA the levels of antibodies fluctuated during the period of study and were not significantly different at the end. There was no relationship between levels of antibodies to type II collagen and any specific index of disease activity, severity of X-ray changes in the hands and feet, or progression over 1 year in X-ray changes. The finding of a decline in levels of antibodies to denatured type II collagen in inactive RA suggests that the anticollagen response is an integral component of the rheumatoid process and could have a primary or secondary role in pathogenesis.