The proportions of chromosomal genotypes associated with the Turner syndrome genotype (excluding those with a Y chromosome) in embryonic and fetal deaths, in fetuses diagnosed prenatally, and in living individuals were reviewed. The ratio of apparent non-mosaic 45,X to 45,X/46,XX mosaics was notably higher in a New York City series of embryonic and fetal deaths, 13.5 to 1, than in living individuals reported to the New York State Chromosome Registry, 3.6 to 1. The ratios of 45,X cases to those with 46,Xi(Xq) was 5.7 to 1 in living individuals, but was 112 to 0 in embryonic and fetal deaths, an even greater disparity, indicating the marked fetoprotective effect of more than one dose of some locus or loci on the long arm of the X chromosome. The results of review of data pertinent to the livebirth prevalence of the (apparent non-mosaic) 45,X genotype suggest a rate of about 5.7 per 100,000 livebirths (11.8 per 100,000 females) with 95% confidence limits of 2.6 per 100,000 to 10.8 per 100,000. The rate in fetuses diagnosed prenatally is 8/27,202, about 30 per 100,000. As a large proportion of these, perhaps 75%, would undergo spontaneous fetal death if not terminated electively, these figures are consistent with the direct estimate of livebirth rates. The rate of all those with X chromosome abnormalities (with a Y chromosome) associated with signs or symptoms that eventually lead to referral for cytogenetic study was estimated at a minimum of 10.7 per 100,000 livebirths, (22.2 per 100,000 females).