TP53 abnormalities in breast carcinomas and inherited TP53 changes in breast cancer patients and in Li-Fraumeni-like families were looked for. Tumours were screened for mutations in the TP53 gene by means of the PCR-CDGE method followed by PCR and direct sequencing. Allelic loss was determined by polymorphic markers, by comparing normal and tumour DNA. Abnormal protein expression was examined by immunohistochemical staining. TP53 abnormalities in the tumours were examined in relation to genetic instability, clinical data and family history. Genetic instability was studied by detection of oncogene amplification, allelic loss, karyotype analysis and fluorescent in situ hybridization, FISH. Our studies showed that TP53 abnormalities were significantly associated with amplification of the erbB2 oncogene and allelic loss on chromosome 17. Chromosomal abnormalities were also significantly more common in tumours with TP53 abnormalities. Looking at clinical data we found significant association between TP53 abnormalities and poor prognosis.