Recent discoveries in the field of molecular-genetic research make it possible to detect an increased genetic risk of tumours, because several genes are linked to hereditary forms of breast cancer. The breast cancer gene BRCA1, located on chromosome 17q, is quantitatively the most important gene so far. A BRCA1 gene mutation is estimated to occur in 1-3 per 1000 women in the general population, i.e. in about 10,000 women among the 4 million Dutch women aged 25-55 years. In this study experiences are described concerning oncologic, clinical-genetic and psychologic aspects in the first Dutch family in which a BRCA1-gene defect was detected with the corresponding hereditary breast/ovarian cancer syndrome. Of the relatives 88% participated in the genetic family study and 76% wished to be informed on the individual DNA-test results. From the first-degree relatives of the breast cancer patients 54% appeared to be gene mutation carrier. The detection of a gene mutation in a woman could make her decide to undergo preventive mastectomy and (or) ovariectomy, besides regular breast examination and mammography. Surgeons and radiotherapists, the group of doctors who treat primary breast cancer, have to anticipate more radical operations with regard to breasts in this selected group of (future) patients. Detection of the gene may also have consequences for family planning. Identification of carriers of the gene mutation can lead to a selection of women with increased risk of breast cancer. Primary or secondary preventive measures, early diagnostic management and regular examination may lead to a decrease in death from breast cancer.