Apoptosis and proliferation are processes associated with the development and progression of breast cancer. The sensitivity of tumour cells to the induction of apoptosis depends on the balance between pro- and anti-apoptotic proteins. The expression of Bak and Bcl-2 was examined using an immunohistochemical method in 71 primary breast cancers. Furthermore, Bcl-2 and Bak were assessed in the normal mammary gland as well as in benign mammary dysplasia adjacent to breast cancer. Positive immunostaining for Bcl-2 was observed in 77.8% of cases of normal breast epithelium (NBE), 93% of benign dysplasia without intraductal proliferation (BBD) as well as in 94% of intraductal proliferative lesions of the breast (BIPL). Expression of Bak was detected in 39% of cases of NBE, 45% of BBD and in 67% of BIPL. In breast cancer Bcl-2 and Bak expression was found in 83% and 70% of the cases studied, respectively. Increased Bcl-2 expression in primary tumours significantly correlated with favourable prognostic factors, namely pT1, G2 and lack of metastases to the regional lymph nodes (p < 0.01, p < 0.03, p < 0.02, respectively). There were no relationships between Bak and the clinicopathological features studied, but our results indicate changes in the expression of Bak during breast cancer development and progression. It would appear to be important to assess and compare pro- and anti-apoptotic proteins between normal mammary gland, benign mammary dysplasia and the primary tumours of breast cancer. This knowledge should be helpful in understanding breast cancer development and progression.