Von Recklinghausen neurofibromatosis is characterized by a relatively large proportion of apparently nonfamilial cases, presumed spontaneous mutations. This paper analyzes the distribution of paternal and maternal ages for 187 patients with von Recklinghausen disease representing the first definite case in their respective families. Mean paternal age was 32.8 years and mean maternal age was 27.4 years, both being significantly greater than for control populations (P equal to or less than .001). The advanced paternal age was not accounted for by the increase in maternal age. The methodology of controlling the general population paternal ages for each patient's birth year is described.