The prognostic significance of p53 gene abnormalities was investigated in 919 primary breast-cancer patients. p53 expression and tumour-cell proliferation fraction determined by MIB-1 count, p53 exon 5 and 6 mutations and HER-2/neu oncogene amplification were detected by immunohistochemistry, PCR-SSCP and slot-blot hybridization, respectively. Increased MIB-1 count, p53 expression, HER-2/neu oncogene amplification and p53 mutations were detected in 33%, 29%, 10% and 8% of tumours, respectively. Statistically significant associations were observed between p53 expression or MIB-1 count and age below 50 years, high-grade tumours, medullary carcinomas, and absence of hormone receptors. p53 mutations were associated with increased MIB-1 count, HER-2/neu oncogene amplification and absence of hormone receptors, but not with age, tumour size or grade, histological subtype, or the number of axillary nodes involved. After a median follow-up of 66 months, p53 expression was observed to be associated with significant increases in risk of both relapse and death from breast cancer, but not after adjusting for the effect of other parameters. In these analyses, MIB-1 count, and not HER-2/neu oncogene amplification, was an independent predictor of prognosis. In node-negative patients, only p53 exon 5 and 6 mutations and MIB-1 count were associated with a statistically significant increase in risk of death from breast cancer, independent of tumour size and ER concentration. We conclude that tumour-cell proliferation fraction, as measured by MIB-1 count, is the most useful parameter of breast-cancer prognosis, with the exception of ER, tumour size and the number of axillary nodes involved.