Data from a chromosome examination of 14,069 consecutive newborn infants is presented. Successful karyotypes were obtained on 13,939 babies using short-term blood cultures and conventional staining methods. Of those, 13,645 babies had normal chromosomes; 64 (0.46%) had a major chromosome abnormality; and 230 (1.65%) had a marker chromosome; giving a total of 294 (2.11%) babies with a major chromosome abnormality or distinctive marker chromosomes. Six male babies with sex chromosome abnormalities had a 47,XXY and four a 47,XYY karyotype, and three were mixoploids. Five female babies had a 47,XXX karytotype and two were mixoploids. There were three babies with ambiguous external genitalia, all with normal karyotypes. Fourteen babies had 21-trisomy; there were three 18-trisomics and one 13-trisomic. The mother of one 18-trisomy baby had a balanced (18;21) translocation. Twenty-four infants had a balanced chromosome rearrangement. Eleven of these were reciprocal and thirteen were Robertsonian translocations. One baby had an unbalanced derivative chromosome resulting from an 18;11 insertion. Two infants with additional unidentified fragments were detected. Two hundred and thirty babies (1:60) carying distinctive chromosome variants were detected. The commonest variant was the Yq+ among males (0.89%). Other common variants involved the short arms of the D and G groups (0.32% and 0.57%, respectively) 16q+ (0.09%), and 1q+ (0.04%). The results of the present study when combined with five other comparable studies, thus comprising a total of 46,150 newborn infants, indicates that the frequency of major chromosome abnormalities is between 1:150 and 1:200 live-born babies. This represents a small proportion of all conceptuses with chromosome abnormalities, which has been estimated as being approximately 1:20. It is thus clear that chromosome abnormalities form a major part of the genetic load carried by the human population. The development of chromosome banding techniques already has increased, and with further increase, the complexities of human cytogenetics and may reveal many additional rearrangements undetectable by conventional methods.