"An ounce of prevention is worth a pound of cure" In medicine the truth of this statement is so self-evident that it is simply taken for granted; and yet it has become mere lip-service for many a doctor, since his work is almost exclusively concerned with the treatment of those who are already ill. This applies not only to the treatment of patients but even more to that of entire families. Many doctors are as yet unaware that the appearance of serious, sometimes fatal diseases can be avoided by preventing the concep tion of sick human beings. Our knowledge of genetics permits the relatively accurate prediction, based on statistical probability, of the recurrence of genetic defects (anomalies) and diseases within families. Our patients are frequently aware that such predictions are pos sible. In an effort to prevent the birth of defective children they try to inform themselves. However, in the practice of the individual doctor this sort of inquiry does not occur with such frequency that he is forced to concern himself systematically with these problems.