The early course of newly diagnosed RA among young adult patients (16-44 yr) is described from results of an ongoing study with a mean follow-up of 3.4 yr. Study diagnosis was based on the judgement of experienced rheumatologists, and data on several hundred variables were obtained on entry and annually for the purpose of defining patterns of onset and course of disease. Race and sex factors, as well as certain entry manifestations, e.g., RF, were found to correlate with onset and course patterns. Females, and especially white females, had significantly greater numbers of swollen upper extremity joints than males at entry and at last observation, with increased likelihood of developing bone erosions. At entry, RF positive patients differed only on few articular manifestations from RF negative patients, but had a higher frequency of positive ANA at entry and more subcutaneous nodules and bone erosions during follow-up. Seropositive white females at entry had significantly more swollen upper joints than their seronegative counterparts, but with no difference found at last follow-up. White females of each serogroup had more joint involvement at last examination than patients of other race-sex groups. Males had more acute onset, especially under age 30, with significantly greater improvement in arthritis and in ESR than did females. The majority (55%) of patients entered as seropositive, converted to seronegative during follow-up, and no correlation of either joint swelling or erosions was noted with this phenomenon. At last visit, RF positively did not correlate with bone erosions, but patients developing bone erosions had higher frequency of ANA and higher mean serum complement levels at last examination. The following entry factors were found to correlate significantly with a better outcome: maleness, acute onset under age 30, less swollen upper joints, and negative RF. Type of drug therapy tended to reflect severity of arthritis, rather than vice versa, and functional capacity improved significantly from entry to last evaluation in both males and females, even though the latter had stable or progressive arthritis. Further study is necessary over long intervals and in wider age range to more adequately interpret the biologic implications of findings of this ongoing study. A better understanding of the pathogenesis of RA may be derived from critical studies of the contribution of host factors, e.g., sex, and other variables predisposing to the development of RF positivity (and ANA), subcutaneous nodules, and bone erosions, particularly in systematic ongoing studies of patients with early diagnosed disease.