Carcinoid tumor of the kidney is a rare neoplasm of uncertain histogenesis. Attempts to elucidate its cell of origin have been made, but there is a lack of experimental proof. We present a case of primary renal carcinoid tumor with a characteristic molecular abnormality and discuss its histogenetic implications. Histologic, immunohisto-chemical, and electron microscopic analyses revealed features typical of carcinoid tumor, and DNA flow cytometric analysis showed diploid pattern. Molecular genetic studies of informative WT1, p53, and 3p21 loci revealed loss of heterozygosity only at the D3F15S2 locus (3p21 telomeric). The similarity between the molecular abnormality in the present case and that in most renal cell carcinomas suggests a possible common genetic event in the genesis of these neoplasms.