This paper describes the investigation of nuclear DNA content and p53 immunoreactivity in normal mucosa (n = 25), mildly (n = 15), moderately (n = 28) and severely atypical (n = 22) colorectal adenomas and in colorectal adenocarcinomas (n = 116). Twenty-seven per cent of the mildly atypical, 43% of the moderately, 77% of the severely atypical adenomas and 91% of the colorectal carcinomas were distinctly aneuploid. In the aneuploid lesions p53 immunoreactivity was not observed in mildly atypical adenomas, whereas 17% of the moderately atypical, 24% of the severely atypical adenomas and 66% of the adenocarcinomas were p53 positive. None of the diploid lesions were p53 immunoreactive. These data are interpreted to indicate that genomic instability as reflected by crude aneuploidy occurs early during genesis of colorectal carcinoma and represents a high risk factor for p53-gene mutation.