We measured the impact of rheumatoid arthritis (RA) and osteoarthritis (OA) by comparing the activities of patients with these illnesses to controls matched for age, sex, and community of residence. Our results indicate that patients with RA experience more losses than controls in every domain of human activity and that patients with OA experience more losses in the performance of household chores, shopping and errands, and leisure activities. The methods described here provide a simple, reliable way to assess the impacts of illness in the same terms for all dimensions of human activity.