This paper explores usage of the concept ofabnormality in medical genetics and proposesdirectives for more careful usage of this concept.The conceptual difficulties are first explored, thena model is developed to assess actual usage, followedby analysis of a sample of genetic textbooks andgenetics literature. It appears that fact andvaluation are often intermingled, that referencestandards used to define 'genetic abnormalities' areoften not clear and that the concept of abnormality isoften used independent of the degree of certainty withwhich the altered genetype develops into (...) a (seriously)harmful phenotype. On the basis of these findings itis argued that more restraint and more careful use ofthe concept of genetic abnormality of medical geneticsis appropriate as well as more agreement on the use ofreference standards. (shrink)