In a systematic study of bladders from consecutively cystectomized patients, 40 primary urinary bladder carcinomas were examined with regard to the type of differentiation. Metaplastic areas were found in 24 of the tumours. A positive reaction of keratin to Kreybergs stain was required for the definition squamous metaplasia. Squamous areas were found in 16 of the tumours. In 4 tumours there was, in addition to squamos metaplasia, glandular metaplasia which was defined as definite glandular tissue with dysplasia of the epithelium. In 4 tumours, glandular metaplasia was also present in addition to the urothelial carcinoma. In 18 cases, metaplastic changes were present in the luminal portion of the tumour from which the biopsy material and desquaminated material originated. The occurrence of metaplasia was related to the degree of differentiation of the urothelial part of the tumour. Metaplastic changes were most frequent in the poorly differentiated tumours. The biopsies obtained pre-operatively permitted of the diagnosis metaplasia being made in 15 cases, while the cytological material suggested metaplastic changes in 4 cases only. The biological function of metaplasia is unknow. The possibility that these areas will react differently to radiation and chemotherapy is present and the frequency of such changes would suggest that their importance should be studied by their registration as mixed forms as stated by WHO.