Morphological similarities between renal oncocytomas and 'oncocytic' renal cell carcinomas (RCCs) make a differential diagnosis in many cases difficult. A series of 41 renal oncocytomas has been analysed by microsatellite markers from chromosomes 1, 2, 3p, 6q, 8p, 9, 10, 13q, 14q, 17, and 21, alterations of which are known to be involved specifically in non-papillary and chromophobe RCCs. Only eight of the 41 renal oncocytomas showed loss of heterozygosity (LOH). LOH at chromosomes 1 and 14 occurred in four tumours each and at chromosomes 2, 8, and 9 in one tumour each. Combined LOH at chromosomes 1, 9, and 14 and also at chromosomes 1 and 14 occurred in one case each. No LOH was seen at any other genomic sites. The lack of combination of LOH at specific chromosomal sites differentiates renal oncocytomas from other renal cell tumours with overlapping phenotypes. Applying the microsatellite assay described here, the diagnosis can be established within 2 days, from fresh as well as from paraffin-embedded material.