Regarding the determination of genetic risk factors for serious diseases, the main question for a patient's young relatives is 'to know or not to know'. The answer depends among other things on the assessment of the severity of the disorder and the magnitude of the risk in relation to the population risk and on the availability of therapeutic or preventive measures. Experience with the requests for and the coping with results of the presymptomatic DNA test for untreatable neurodegenerative chorea of Huntington show that expectations about the effect of predictive DNA studies are often different from reality. A problem of a different nature arises in multifactorial diseases such as mammary or colonic carcinoma, because irrespective of the DNA study it remains uncertain whether or not the disease will occur. Nevertheless more than half of the healthy relatives of a patient with breast cancer requests DNA testing and a large majority of the proven carriers of a BCRAI and BRCA2 mutation chose for bilateral mastectomy and oophorectomy. Many questions about psychosocial consequences of predictive DNA testing remain to be answered: what will be the effect of early medicalization, how will relations be affected and which effect will carriership of genetic risk factors have on behaviour? Dutch legislation provides adequate guarantees against the use by third parties of results of genetic testing. The question remains, however, how future internationalization will work out in this respect. The possibly far-reaching social and psychological consequences of genetic research make some people feel that restraints should be imposed on this research. However, there are more grounds for curiosity and enthusiasm to constantly find new solutions for the new problems.