AbstractSeizure‐prone patients may be chronically treated with diphenylhydantoin, a mitogenic agent excreted in the urine. Chronic ingestion of diphenylhydantoin by 64 rats failed to demonstrate vesical epithelial tumor formation under conditions which demonstrate the tumorigenicity of other vesical carcinogens. Vesical neoplasms occurring in patients chronically ingesting diphenylhydantoin are probably co‐inciden