Small cell carcinomas of the urinary tract are rare, but lethal. We report 3 cases of primary small cell carcinoma of the kidney, urinary bladder and prostate with light microscopic, immunohistochemical and electron microscopic findings. One patient with small cell carcinoma of the prostate died of disseminated disease 2 years after diagnosis and another patient with small cell carcinoma of the urinary bladder was free of tumour after 6 months. A partial remission was induced in the third patient with distant metastases of small cell carcinoma of the kidney by using chemotherapy protocols similar to the drug regimens for small cell carcinomas of the lung; the patient survived for 5 months. Immunohistochemical studies revealed the absence of argyrophilic immunostaining of tumour cells in all 3 cases, positive staining for keratin in 2 and staining for neuron-specific enolase in all 3. In the third patient, reactivity for prostate-specific antigen was negative. Dense-core, membrane-bound granules were identified in the cytoplasm of 2 patients. The paraneoplastic syndrome was not found, indicating that in considering the occurrence of ectopic hormones, specific cytoplasmic granules of origin need not be implicated. Recognition of this distinct entity requires full consideration of morphological, immunohistological, ultrastructural and biological features. In order to define the origin of this tumour more clearly and to evaluate the effectiveness of chemotherapy, larger series of patients are needed.