Forty-eight patients with erythema chronicum migrans (ECM) were studied prospectively for 6 to 18 months. Twenty-six patients had no later symptoms, but 22 subsequently developed Lyme arthritis and 9 of them also experienced neurologic abnormalities. Eighty-seven percent of patients with active ECM followed by subsequent involvement had cryoglobulins containing IgM compared to only 13% of those with active ECM and no later symptoms. The former group also had significantly lower IgG, C3 and C4 levels. Sixty-seven percent of patients still had serum cryoglobulins when neurologic disease was most active, and 45% had them when joint symptoms were most severe, but only 11% continued to have small amounts in remission. The number of patients who continued to have serum cryoglobulins with recurrent attacks of arthritis decreased with time. In contrast, patients always had cryoglobulins in joint fluid, a finding Lyme arthritis shares with rheumatoid arthritis. The cryoprecipitates from 2 of 10 patients contained particles with internal structure, but their viral nature is problematic. All components of antisera obtained from goats and rabbits immunized with cryoglobulins were absorbed by normal human sera. The amount of IgM in cryoglobulins correlated directly with serum IgM, which generally rose during exacerbations and fell during remissions; serum IgG and IgA moved conversely. Thus, IgM was an important correlate of clinical disease activity and IgG or remission.