The defect of the enzyme heparan sulfamidase is the cause of Sanfilippo A syndrome (mucopolysaccharidosis IIIA) which is an autosomal recessive inherited disease. Three children of a marriage couple who attended our genetical counselling died of this disease which at the moment is incurable. At the 10th week of the following pregnancy chorion villi analysis was carried out and normal values of the enzyme heparan sulfamidase were obtained. In view of this it was decided that pregnancy should continue. At the moment the child is one year old and clinical and laboratory findings also show that she is not affected.