All 5 cross sectional population based studies which included evaluation of all subjects for both rheumatoid factor (RF) and rheumatoid arthritis (RA) were reviewed. RF was found in only 19-33% of individuals who met the 1958 ARA criteria for RA. Many individuals identified in these studies met criteria for only "probable" or "possible" RA according to the 1958 criteria, and might not meet the 1987 criteria. However, RF was found in only 26-60% of subjects who met criteria for "definite" RA in the studies that included this information. Population based studies of RA often have been interpreted as applicable to clinical RA patients, which may explain in part different views of RA as both a mild and a progressive disease.