The aim of the study was to investigate the relationship between the expression of human epidermal growth factor receptor 2 (HER2) and activator protein 2 (AP-2), as well as the prognostic significance of HER2 in breast cancer. HER2 and AP-2 expressions were immunohistochemically (IHC) analysed in a large prospective, consecutive series of 425 breast cancer patients diagnosed and treated between 1990 and 1995 at the Kuopio University Hospital, Kuopio, Finland. In a subset of patients (n = 71), the gene amplification status was examined by using a chromogenic in situ hybridisation (CISH) analysis. Expression of HER2 was studied in relation to AP-2, clinicopathological parameters and patients' survival. Pathological membranous overexpression of the HER2 receptor was seen in 13% of the carcinomas, which was related both to gene amplification (78% of the cases) and high nuclear expression of AP-2 (67%, P = 0.007). HER2-positivity was most often seen in carcinomas having both high nuclear and high cytoplasmic AP-2 expression (P < 0.001). In the univariate survival analyses HER2-positivity predicted a shorter recurrence-free survival (RFS) (P < 0.0001) and a shorter breast cancer-related survival (BCRS) (P = 0.0063) in the whole patient group, as well as in the subgroup of node-negative patients. In the node-positive patients, HER2-positivity predicted only a shorter RFS. Combined expression of HER2 and nuclear AP-2 resulted in the separation of four groups with different prognoses, the best prognosis being for patients in the HER2-/AP-2+ group and the worse prognosis for those in the HER2+/AP-2- group. In the multivariate survival analyses, HER2-positivity independently predicted a shorter RFS in the whole patient group (P = 0.0067), as well as in the subgroup of node-positive patients (P = 0.0209). In conclusion, pathological membranous overexpression of the HER2 receptor in breast cancer is related both to gene amplification and a high AP-2 expression. Combining HER2 and AP-2 expressions may provide valuable information on the prognosis of breast cancer patients.