This study will compare the safety and effectiveness of two drugs-methotrexate and mycophenolate mofetil (MPM)-in preventing disease recurrence in patients with Wegener's granulomatosis and related inflammatory blood vessel disorders. The standard treatment for these conditions is combination drug therapy with prednisone plus cyclophosphamide. However, although most patients improve on this therapy and achieve disease remission, many experience a relapse (return of the disease) some time after therapy is stopped. Also, these drugs can produce serious side effects during treatment. This study will test a new treatment regimen to try to maintain disease remission in these patients with minimal side effects.