The investigators have found that patients with PR3-ANCA-associated vasculitis who remain C-ANCA positive after induction of remission have an increased risk to experience relapse of disease (MC Slot et al. Arthritis Rheum. 2004 15;51(2):269-73). Therefore they will test whether relapse risk in these patients can be reduced by extending maintenance therapy at the cost of acceptable therapy related toxicity. After induction of stable remission, ANCA will be measured by IIF. C-ANCA positive patients will be randomized for either standard therapy with azathioprine (until 18 months after diagnosis), or longterm azathioprine maintenance therapy (until 48 months after diagnosis).