The introduction of cyclophosphamide (CyP) as a treatment for Wegener's granulomatosis (WG) and microscopic polyangiitis (MPA) has been among the most significant contributions in vasculitis. Prior to the introduction of CyP, WG was a uniformly fatal disease, with mortality occurring within 5–12 months from pulmonary or renal failure or from infection due to glucocorticoids. In 1973 Fauci and Wolff, at the National Institutes of Health, published their experience with a regimen that combined CyP and prednisone in which disease remission was seen in 12 of 14 patients. Long-term experience with CyP provided even greater evidence for its efficacy in which an 80% survival rate was seen, with 91% of patients having significant improvement and 75% achieving complete remission. However, extended follow-up also demonstrated that disease relapse occurred in at least 50% of patients and that 42% experienced morbidity solely as a result of treatment. These observations showed that while CyP was life-saving, it did not prevent relapse and was associated with significant toxicity such that safer means to induce remission needed to be pursued. Strategies aimed at reducing exposure to CyP have included intermittent administration, induction–maintenance regimens and avoidance of CyP for non-severe disease. Recently, the introduction of rituximab has raised important questions regarding the place of CyP in the treatment of WG/MPA. This paper examines the past, present and future of CyP through a review of its efficacy and safety.