Although a clear distinction between the Werner syndrome and premature aging was made in our review published in 1966, repeated allusions in the literature indicate either an ambivalence about this relationship or a belief that the two conditions are fundamentally related to one another. Therefore, the criteria for premature aging originally used have been reexamined and found to be still acceptable. Evaluation of the features of the Werner syndrome in the light of these criteria once again indicates that there is no substantial evidence to support and considerable evidence to oppose equating the Werner syndrome with aging. This conclusion applies not only to the Werner syndrome in toto, but also to those aspects of the syndrome considered to qualify it as a segmental progeroid syndrome and to its characteristic cellular abnormalities (severely limited in vitro proliferation and variegated translocation mosaicism). The Werner syndrome should not, therefore, be forced into the mold of premature aging but should be studied on its own merits as a condition which may provide us with clues to the pathogenesis of many important problems.