This study will examine the safety and effectiveness of the drug mycophenolate mofetil (MPM) in treating Wegener's granulomatosis and related inflammatory vessel diseases. Blood vessel inflammation in these patients may involve different parts of the body, including the brain, nerves, eyes, sinuses, lungs, kidneys, intestinal tract, skin, joints, heart, and other sites. The more severe the involvement, the more likely the disease will be life-threatening. Standard treatment consists of combination drug therapy with prednisone and a cytotoxic agent-usually cyclophosphamide or methotrexate. However, some patients in whom this treatment is initially successful have a disease relapse; other patients cannot take the medications because of other health problems or because of severe side effects of the drugs.