A classification of a group of malformations of the extremities is given and an underlying common pattern is developed from a survey of 693 deformed limbs. It is characterised by a certain reduction tendency of the affected bones and the malformed extremity, an axis of malformation and an interdependence between the proximal and peripheral parts of the deformed limbs. The group, called dysmelia, has a common morphology, but morphologically identical types have been seen caused by thalidomide and as hereditary or sporadic cases. An attempt has been made not only to name and classify these deformities but also to clarify the underlying principles of their morphology. This will enable the teratologist to see the pattern of these malformations in comparison with that of normal development of the limbs and of experimentally induced limb defects.