Fifteen patients with Wegener's granulomatosis, polyarteritis nodosa, or isolated angiitis of the central nervous system were treated with cyclophosphamide according to a widely used regimen. Seventeen clinical episodes of infection were observed over 201 patient-months of cyclophosphamide therapy, and 2 patients died of pneumonia. Notably, neither the incidence of leukopenia nor the dosage or duration of cyclophosphamide or corticosteroid therapy correlated well with infection, which occurred most frequently in men over 60 years of age. Patients with Wegener's granulomatosis appeared to be at greater risk of infection than those with the other forms of vasculitis. These results suggest that this treatment regimen may not be as safe as was previously thought.