A 4-year-old Jewish boy presented with dysuria, urinary dribbling, increased urinary frequency, and new onset of diurnal enuresis. An infiltrating solid mass involving the entire bladder wall was found. Biopsy revealed "tumor-forming" eosinophilic cystitis, a rare bladder lesion of unclear cause. Antitoxocariasis treatment was unsuccessful. High-dose corticosteroids failed. The child's clinical condition and bladder sonographic findings continued to deteriorate. Treatment with cyclosporin A was given for 8 months, with a complete clinical, radiologic, and histopathologic cure and no side effects. Two years of follow-up showed a complete recovery.