A two year experience of DNA diagnosis for NF1 is presented. Twenty-three NF1 families have been analysed using 11 closely linked and intragenic markers. Prenatal testing was undertaken for six families; 11 affected subjects and their partners wished to know if they would be informative for future prenatal testing, seven of whom are so far fully informative. Presymptomatic testing was done for six subjects. Despite the availability of a series of closely linked markers, informativeness could not be achieved in all of the families tested.