Two sets of twins are presented, in which one child in each set has arthrogryposis multiplex congenita and the other child in each set is entirely normal. These are sets of identical twins, as demonstrated by both clinical and serological data. The disease in these instances could not have been genetically produced, since there is no deformity in the monozygotic twin. Doubt is expressed whether maternal infection or intra-uterine environment could affect one child and not the other. No current concept of etiology adequately accounts for the manifestations of the disease.