Current conventional therapies for ANCA-associated vasculitis (AAV) are associated with high incidences of treatment failure, disease relapse, substantial toxicity, and patient morbidity and mortality. Rituximab is a monoclonal antibody used to treat non-Hodgkin's lymphoma. This study will evaluate the efficacy of rituximab with glucocorticoids in inducing disease remission in adults with severe forms of AAV (WG and MPA).

The study consists of two phases: a 6-month remission induction phase, followed by a 12-month remission maintenance phase. All participants will receive at least 1 g of pulse IV methylprednisolone or a dose-equivalent of another glucocorticoid preparation. Depending on the participant's condition, he or she may receive up to 3 days of IV methylprednisolone for a total of 3 g of methylprednisolone (or a dose-equivalent). During the remission induction phase, all participants will receive oral prednisone daily (1 mg/kg/day, not to exceed 80 mg/day). Prednisone tapering will be completed by the Month 6 study visit.

Next, participants will be randomly assigned to one of two arms. Arm 1 participants will receive rituximab (375 mg/m2) infusions once weekly for 4 weeks and cyclophosphamide (CYC) placebo daily for 3 to 6 months. Arm 2 participants will receive rituximab placebo infusions once weekly for 4 weeks and CYC daily for 3 to 6 months. During the remission maintenance phase, participants in Arm 1 will discontinue CYC placebo and start oral azathioprine (AZA) placebo daily until Month 18. Participants in Arm 2 will discontinue CYC and start AZA daily until Month 18. Participants who fail treatment before Month 6 will be crossed over to the other treatment arm unless there are specific contraindications.

All participants will be followed for at least 18 months. Initially, study visits are weekly, progressing to monthly and then quarterly visits as the study proceeds. Blood collection will occur at each study visit.