Background: Relapses of granulomatosis with polyangiitis and microscopic polyangiitis, collectively ANCA-associated vasculitis (AAV), are common and important events. Few large, international studies describe risk factors for relapse of AAV. Methods: Relapses occurring in participants with severe AAV enrolled in PEXIVAS, an international, 2-by-2 factorial trial of induction treatments, were studied. The primary outcome of this analysis was relapse occurring at least 90 days after randomization. Candidate predictors included baseline participant and disease characteristics. The association between relapse and the candidate predictors was assessed using time-to-event models incorporating death as a competing event using the Fine and Gray method. All models were adjusted for induction therapies. Results: Over a median follow-up of 2.93 years, 150 (23.3%) participants experienced at least one relapse (incidence rate 7.4 per 100 patient-years). The median time to relapse was 483.5 days (interquartile range 198-920). The most common manifestations of disease at relapse were renal (58.0%), constitutional (38.7%), and ear/nose/throat (31.3%). Baseline characteristics associated with an increased risk of relapse included: PR3-positive ANCA, skin involvement, and non-hemorrhagic lung involvement (Table 1). Characteristics associated with a lower risk of relapse included female sex and receipt of dialysis. Discussion: Relapses remain common among patients with severe AAV. Identifying those most at risk of relapses may help plan treatments and monitoring.