Bax and Bc12 are functionally antagonistic proteins which control apoptosis, whose expression in human tumours could be of prognostic value. We evaluated Bax and Bcl2 expression in 239 breast carcinomas (99 N0, 140 N1/2) with long term follow-up (median 79 months, range 11-140) in relation to clinico-pathologic parameters, clinical outcome, adjuvant therapy and expression of oestrogen receptor protein and p53. The prognostic value of Bax was investigated in the whole series of patients and in subgroups of homogeneously staged and treated patients (i.e., node-negative, N1/2 CMFtreated, N1/2 tamoxifen-treated). Bax immunostaining was cytoplasmic and heterogeneous. Cases were scored as Baxpositive if there were more than 20% reacting cells. High Bax expression was associated with positive nodal status (p β«Ψβ¬ 0.03) and high Bcl2 expression (p β«Ψβ¬ 0.01) and was more frequent in high-grade tumours. In the node-negative subgroup, Bax expression was associated with small tumour size. No association was seen with other parameters or with clinical outcome in any subgroup of patients. Since the apoptotic rate of a tumour is influenced by the ratio Bcl2/Bax, we investigated the combined effects of Bax and Bcl2 expression in relation to clinical outcome. However, no differences in survival were seen in the Bcl2-negative and Bcl2-positive groups when they were subdivided on the basis of the level of Bax expression and vice versa. In experimental systems, p53 is a direct transcriptional activator of the human bax gene. However, we could not observe any relation between Bax and p53 expression. We investigated whether the combined p53/Bax expression could have any prognostic value since it is predicted that tumours with normal p53 expression and concurrent high levels of Bax should be less aggressive and more susceptible to therapy. However, while p53 itself was of prognostic value, Bax expression was not related to prognosis in p53-negative or in p53-positive groups.