One hundred patients who underwent jejunoileal bypass for obesity were followed for a mean period of 2 1/2 years. Four patients developed a clinical illness that resembled a systemic form of tuberculosis during the first postoperative year. This incidence exceeds that found in the general population by sixtyfold. Any patient with jejunoileal bypass who develops an illness with accelerated weight loss, enlarged lymph nodes, and unexplained fever with chills should be suspected of having tuberculosis. Aggressive diagnostic measures are required. Treatment with isoniazid and ethambutol at usual doses can be successful, but blood levels should be measured to confirm adequacy until additional information becomes available.