The direct study of human chromosomes has only recently become of significance in medicine. This has been made possible by several important advances in cytological technique, leading to the unexpected discovery in 1956 by Tjio and Levan 1 that the human chromosome number was 46 and not 48. The latter had been the accepted diploid number for over 30 years. Techniques have continued to improve, and in addition to confirming the new diploid number, a reasonably accurate analysis of the normal human chromosome complement has been accomplished. For the first time, the study of human chromosomes in relation to disease has become practical. Since January, 1959, specific abnormalities in the chromosome complement have been shown in several congenital conditions, including Mongolism and two types of sex anomaly. Before considering the implications of these findings, an account of the investigations which have led to them seems appropriate. Normal Chromosome Complement Although