Tumour eosinophilia is an uncommon but striking phenomenon which has been found in many tumours, mostly of large cell type or squamous differentiation. The incidence, appearance and importance of tumour eosinophilia in the bladder are described. Eosinophilia is commoner in deeply invasive tumours and in tumours showing squamous metaplasia. Transitional cell carcinomas with eosinophilia have a better prognosis than those without, but this improvement is not seen in squamous cell carcinomas of the bladder. When eosinophilia is found on superficial biopsies of a bladder tumour, the possibility of muscle invasion should be considered.