The acceptability of presymptomatic testing in 21 people at 50% risk for the APP-692 mutation causing presenile Alzheimer's disease or cerebral haemorrhage resulting from cerebral amyloid angiopathy (FAD-CH), and in 43 people at 50% risk for hereditary Pick disease (HPD) was assessed. Neither group differed in demographic variables. Thirty-nine people (64%) in the whole group would request presymptomatic testing if it were clinically available, although two-thirds did not yet feel ready to take it. The most important reasons in the HPD and FAD-CH group for taking the test were: to further basic research (42% and 47%, respectively), informing children (47% and 50%, respectively), future planning (29% and 47%, respectively), and relieving uncertainty (46% and 27%, respectively). The most commonly cited effect of an unfavourable test result concerned increasing problems for spouses (75% and 76%, respectively) and children (61% and 57%, respectively). Most respondents denied that an unfavourable result would have adverse effects on personal mood or relationship. One-third of all respondents favoured prenatal testing where one of the parents had an increased risk for HPD or FAD-CH. Participants would encourage their offspring to have the test before starting a relationship (35%) and before family planning (44%). Thirty-seven percent of the respondents would encourage their children to opt for prenatal diagnosis. People at risk for HPD were significantly more preoccupied with the occurrence of potential symptoms in themselves, compared with those at risk for FAD-CH, reflecting the devastating impact that disinhibition in the affected patient has on the family. Our findings underline the need for adequate counselling and the availability of professional and community resources to deal with the impact of test results in subjects and their relatives.