Eosinophilic cystitis is a rare form of an allergic cystitis. Factors such as food allergens, parasites, and drugs have been implicated in the genesis of eosinophilic cystitis. Associated risk factors include bronchial asthma, atopic diseases, and environmental allergens. Intravesical mitomycin is a reported causative agent. We report the first case of eosinophilic cystitis that developed after intravesical instillation of thiotepa for treatment of superficial bladder cancer.