Sections of 114 renal neoplasms from adults, and 2 renal rhabdoid tumours from children, were examined by an indirect immunoperoxidase method using two antibodies to Tamm-Horsfall protein. Forty-five of the adult neoplasms were also examined with an antibody to proximal tubular brush border. Tamm-Horsfall protein is normally only found in the cells of the thick limb of the loop of Henle, and there are widely divergent reports on its occurrence in renal neoplasms. In the present series, Tamm-Horsfall protein was detected in parts of 31 neoplasms. Four distinctive patterns of cell contained the protein: cells with a paranuclear inclusion typical of rhabdoid tumours; 'plasma rich' cells, which were large cells with cytoplasm that was centrally dense and peripherally clear; eosinophilic cells forming one type of papillary structure; and giant cells. The areas containing Tamm-Horsfall protein did not express markers of proximal tubular brush border, and appeared white to the naked eye, rather than the yellow of typical clear cell carcinomas. Tamm-Horsfall protein can therefore be found in renal neoplasms. The four distinctive patterns of positive cells appear to represent neoplastic phenotypes of thick limb cells. This has implications for the classification of renal neoplasms and for theories of their origin.