This study compares the effectiveness of high-dose cyclophosphamide treatment with the "gold standard" treatment, monthly intravenous (IV) cyclophosphamide, in people with moderate to severe lupus that does not respond to high-dose corticosteroid therapy. We will give patients either IV cyclophosphamide (750 milligrams per square meter of body surface area) monthly for 6 months, followed by quarterly maintenance therapy, or high-dose IV cyclophosphamide (50 milligrams per kilogram body weight per day) for the first four days of the study. Patients will be followed for 24 months after therapy.