Objective.To assess the incidence and the risk factors for zoster in patients exposed to intravenous cyclophosphamide (CYC) for systemic vasculitis or systemic lupus erythematosus (SLE), as well as the protective effect of prophylaxis by valacyclovir (VCV).Methods.This retrospective study included all adults treated by intravenous CYC for SLE or systemic vasculitis between 2011 and 2015 at Toulouse University Hospital, France. Zoster occurrence was recorded using medical chart review, laboratory data, and patient interviews. Univariate Cox models were computed to assess the risk factors for zoster and the protective effect of prophylaxis by VCV.Results.The cohort consisted of 110 patients (81 systemic vasculitis and 29 SLE). During a mean followup of 3.4 years after CYC initiation, 10 cases of zoster occurred, leading to an overall incidence of 27.9/1000 patient-years (95% CI 15.2–50.6); it was 59.4/1000 patients (95% CI 27.5–123.6) during the year after CYC initiation. Four patients experienced persistent postherpetic neuralgia. Probable risk factors were lymphopenia < 500/µl at CYC initiation (HR 5.11, 95% CI 0.94–27.93) and female sex (HR 4.36, 95% CI 0.51–37.31). The incidence was higher in patients with SLE (HR as compared with systemic vasculitis patients = 2.68, 95% CI 0.54–13.26). None of the 19 patients exposed to VCV during the followup developed zoster.Conclusion.The incidence of zoster is high in systemic vasculitis and in patients with SLE exposed to intravenous CYC. CYC may favor postherpetic neuralgia. Prophylaxis by VCV should be considered, particularly in cases of lymphopenia < 500/µl at CYC initiation and during the year after.