Malakoplakia is a chronic granulomatous inflammatory disorder. It is suspected clinically by the presence of chronic infection and diagnosed by histologic examination of affected tissues. Studies of 4 patients with malakoplakia--2 renal transplant recipients, 1 patient with systemic lupus erythematosus, and 1 patient with polymyositis--are reported. All patients were receiving prednisone and azathioprine at the time of diagnosis and had an infection caused by Escherichia coli. Leukocytes from all patients failed to kill Staphylococcus aureus and E coli normally in vitro. Cholinergic agonists had no apparent effect on bacterial killing in vitro or in vivo in the 2 patients examined. Clinically, malakoplakia improved significantly when immunosuppressive therapy was tapered or discontinued, and leukocyte function returned to normal in all 4 patients. The cases reported here and those documented previously suggest that the pathogenesis of malakoplakia and its treatment may not be the same for all patients. Malakoplakia may be more common than previously thought, particularly with the increased use of immunosuppressive therapy.