Eosinophilic cystitis, of which 35 cases have been published, is a recurrent cystitis remarkable for its chronicity and for the importance of haematuria; the urine is usually sterile and blood eosinophilia frequently present; at histology, infiltration of the vesical wall by eosinophils is pathognomonic. In view of the irregular results of cortico-steroid treatment, a search for causative factors is required. Parasitic infestation has been suspected but never actually proven, and an allergic origin probably is frequently missed. We report two cases of eosinophilic cystitis where allergy to penicillin and clometacin respectively could be demonstrated for the first time and in which the condition subsided after withdrawal of these drugs.