This paper reviews the concepts of penetrance and expressivity and examines their application to the specific disorder von Recklinghausen neurofibromatosis. The data suggest that assessments of penetrance among predecessors to probands yield results different from those of assessments of penetrance among descendants to probands. For descendants at risk, penetrance is very close to 100%. For predecessors at risk, extremely variable expressivity may confound estimates of penetrance; as a specific example, a family is described in which two brothers have bona fide von Recklinghausen neurofibromatosis and their mother manifests the neurofibromatosis mutation only as iris Lisch nodules.