Patients with eosinophilic granulomatosis with polyangiitis (Churg-Strauss) (EGPA), non-HBV polyarteritis nodosa (PAN) or microscopic polyangiitis (MPA) without Five-Factor Score (FFS=0)-defined poor-prognosis factors were included in two prospective randomized-controlled trials and were initially treated with corticosteroids (CS) alone. Because some patients required subsequent add-on therapies, inclusion characteristics associated with their use were sought. Add-on treatments (cytotoxic agents, biotherapies, intravenous immunoglobulins and plasma exchanges) were subjected to univariate and multivariate analyses. The study included 193 patients (75 EGPA, 61 MPA and 57 PAN). Mean±SD follow-up was 97.6±39.6months. Subsequent add-on treatment(s) were required for 86/193 patients (24 PAN, 32 MPA and 30 EGPA) because of CS failure (37%), relapse (52%) or CS dependence (10%). Seven-year overall survival reached 90% and was comparable for patients given 0 vs ≥1 add-on therapies (P=0.564). However, the mean Vasculitis Damage Index was significantly higher for the latter: 2.93 vs 1.96 (P<0.001), reflecting more frequent sequelae. Initial mononeuritis multiplex was the only factor significantly associated with add-on therapy requirement in univariate (P=0.008) and multivariate analyses (hazard ratio=1.81 [95% CI: 1.12-2.93]; P=0.02). Although FFS=0 predicts good and comparable overall survival of EGPA, PAN or MPA patients, 45% of them required adjunctive treatments for relapse, CS failure or corticodependence, with most having more frequent initial mononeuritis multiplex and sequelae. These findings support prospective evaluation of initial immunosuppressant use combined with CS to prevent treatment failure, relapses and sequelae in FFS=0 patients with mononeuritis multiplex at diagnosis.