Multiple neurofibromatosis is known to be a genetic disease with the autosomal dominant inheritance pattern. In clinical practice, however, we can hardly ever find a case in which the autosomal dominant inheritance is demonstrable, because sporadic mutation is believed to cause about 50 % of the observed rnultiple neurofibromatosis cases, and because such patients show reduced fertility. The authors observed a family case in which the typical autosomal dominant inheritance could be demonstrated. Among 17 consanguinities of the 3 generations studied, 12 had developed multiple neurofibrornatosis. The presumed reason for the high incidence of the disease in the family studied is 2 fold: 1) The autosomal dominant gene responsible for the disease is highly penetrable. 2) The mutant gene responsible for the disease, for some unexplained reason, was transmitted from patient No. 1 to a.ll of her offsprings, instead of to only half of her offsprings, as would be expected. Multiple neurofibromatosis is known to be a genetic disease with the autosomal dominant inheritance pattern. In clinical practice, however, we can hardly ever find a case in which the autosomal dominant inheritance is demonstrable, because sporadic mutation is believed to cause about 50 % of the observed rnultiple neurofibromatosis cases, and because such patients show reduced fertility. The authors observed a family case in which the typical autosomal dominant inheritance could be demonstrated. Among 17 consanguinities of the 3 generations studied, 12 had developed multiple neurofibrornatosis. The presumed reason for the high incidence of the disease in the family studied is 2 fold: 1) The autosomal dominant gene responsible for the disease is highly penetrable. 2) The mutant gene responsible for the disease, for some unexplained reason, was transmitted from patient No. 1 to a.ll of her offsprings, instead of to only half of her offsprings, as would be expected. Multiple neurofibromatosis is known to be a genetic disease with the autosomal dominant inheritance pattern. In clinical practice, however, we can hardly ever find a case in which the autosomal dominant inheritance is demonstrable, because sporadic mutation is believed to cause about 50 % of the observed rnultiple neurofibromatosis cases, and because such patients show reduced fertility. The authors observed a family case in which the typical autosomal dominant inheritance could be demonstrated. Among 17 consanguinities of the 3 generations studied, 12 had developed multiple neurofibrornatosis. The presumed reason for the high incidence of the disease in the family studied is 2 fold: 1) The autosomal dominant gene responsible for the disease is highly penetrable. 2) The mutant gene responsible for the disease, for some unexplained reason, was transmitted from patient No. 1 to a.ll of her offsprings, instead of to only half of her offsprings, as would be expected.