In a retrospective study the expression of the c-erbB-2 oncogene was determined immunohistochemically in 276 breast cancer samples from 253 patients with the antibody 21N. The follow-up period was between 7 and 12 years. This study showed a trend for an inverse relationship between c-erbB-2 positive tumours and estrogen receptors (ER). A correlation was assessed between c-erbB-2 positive tumours and histological grade, liver metastases as first site of metastases, disease free survival time (DFS) in the second and third year after diagnosis and overall survival time (OST) in the third and fourth year after diagnosis. A trend was seen between c-erbB-2 positive tumours and tumour size. No correlation was found between c-erbB-2 positive tumours and age at diagnosis. The method of operation and lymph node involvement. From this study we conclude that there is a significant difference in prognosis the first years after diagnosis, but this difference seems to vanish in a longer follow-up period of 12 years. This provides us with an explanation for the discrepancies in literature concerning c-erbB-2 expression and prognosis in breast cancer. Some investigators did not show differences in prognosis between positive and negative cases after a long follow-up period whereas investigations with a short term follow-up period up to 2-3 years have indeed established a more aggressive behaviour of c-erbB-2 overexpressionary tumours.