The histogenesis of carcinosarcomas has intrigued pathologists for a long time and remains unresolved. Two main theories have been put forward, one suggesting that they are monoclonal, another suggesting that they are biclonal. Our study examined p53 immunostaining in 17 uterine carcinosarcomas (mixed Müllerian tumours) and found positivity in five (30%). There was no disparity in immunostaining between the epithelial and the stromal components in any of the 17 tumours. This concordance in every tumour would be very unlikely if carcinosarcomas are biclonal. However, it would be expected if carcinosarcomas are monoclonal.