For diseases of well-defined genetic etiology and with onset after birth, the age at onset corresponds to the incubation period of the disease. The lognormal model, as used by Sartwell to study the distribution of incubation periods in infectious diseases (Am J Hyg 1950;51:310-8), was applied in this study to the distribution of the ages at onset of genetic diseases. The literature was reviewed for reports of genetic diseases having frequency distributions of ages at onset. Fourteen diseases with well-specified genetic etiology as well as nine other diseases where the contribution of the genetic component to the etiology is not well defined were studied. A graphic method as well as a goodness of fit test were applied to the different age at onset distributions to assess their conformity to the lognormal model. For most of the genetic diseases that have an underlying biochemical abnormality, the age at onset distributions approximated a logarithmic normal model. In seven series of cases of diseases with an established pattern of inheritance but with no defined biochemical abnormalities, only two showed a good fit to the lognormal model. For diseases with ill-defined genetic etiology or strong environmental influences, these distributions of the age at onset did not fit the lognormal model. In a multifactorial model for disease etiology, the present method may be used as a crude way for differentiating the relative importance of etiologic factors acting before and after birth.