Two children were born in a sibship of four following two early spontaneous abortions. One has Noonan syndrome and the other trisomy 21 mongolism. Their mother, who was 38 and 41 years old, respectively, at the time of their births, had also been exposed to an undetermined dosage of abdominal and pelvic radiation. Since the cause of Noonan syndrome has remained an enigma, we describe these two siblings in the hope of shedding some light on the pathogenesis of this well-defined nosologic entity.