Arthritis is a common complication of small bowel bypass, occurring in 5-2096 of the postsurgical patients. Thirteen patients with arthritis related to jejunoileal bypass were studied. These patients had a symmetrical polyarthritis, and 8 also had extraarticular connective tissue disease manifestations. Immunologic evaluations were done on these patients and on a control group of 12 age-and sex-matched postintestinal bypass patients without arthritis. The incidence of positive ANA, rheumatoid factors, immune complexes, and antibodies to intestinal flora was the same in both groups. Patients in both groups showed similar changes in numbers of circulating T and B lymphocytes. More patients in the group with arthritis than in the control group had elevated IgA levels (38% versus 8%), but the difference was not significant (P > 0.05). This study demonstrates that immunologic abnormalities occur after jejeunoileal bypass irrespective of the onset of arthritis or related symptoms. No specific immunologic abnormalities could be associated with the arthritis occurring after small bowel bypass.