A genetic study of 124 cases of neurofibromatosis was performed. The contingent of probands was mainly represented by a Russian population, most of the individuals being born in the European part of the RSFSR. Both parents of the probands were examined in only 58 cases, the proportion of sporadic cases in this group being 0.79, as compared to 0.77 for the whole group under study. The existing data evaluated by a direct method are not yet sufficient for a decisive estimation of the penetrance, which, however, cannot be under 80%. Segregation analysis of descendants from particular marriages showed a good correspondance to the hypothesis of Mendelian dominance (32 affected children out of 65). These results analyzed together with those obtained by other authors permit an inference on the full penetrance of neurofibromatosis. The genetic interpretation of sporadic cases as a result of new mutations is presented. The prevalence of neurofibromatosis among the 16-year-old youths was evaluated as 12.8 with 10-(5). This value is suggested to be an estimation of the incidence of the condition in the general population, the mutation rate evaluated by a direct method being equal to 4.4 with 10-(5) divided by 4.9 with 10-minus 5. The increased birth order of probands in sporadic cases (against the theoretical expectation) as well as increased paternal age (as compared with controls) were found to be statistically significant (P equals 0.004 and P equals 0.03, respectively) while the difference in maternal ages was statistically insignificant (P equals 0.008). No statistical relationship between sporadic cases and occupational exposure of parents to deleterious chemical and physical factors was found.