We report a rare and, to our knowledge, as yet undescribed type of collision tumour - rectal leiomyosarcoma and prostate adenocarcinoma. Our study also provides the first data on molecular alterations [polymerase chain reaction/loss of heterozygosity (LOH) analysis] of the APC, NF-1, DCC, p53, nm23-H1 and BRCA-1 genes in the two components of the collision tumour. None of the genes examined in this study expressed LOH in the prostate carcinoma component of the collision tumour. By contrast, in the leiomyosarcoma component, LOH was found at the DCC and p53 genes, proving that these two tumours did not arise from the same stem cell but represent two different neoplastic growths.