Over 600 cases with a Y aneuploidy (other than non-mosaic 47,XYY) were reviewed for phenotype/karyotype correlations. Except for 93 prenatally diagnosed cases of mosaicism 45,X/46,XY (79 cases), 45,X/47,XYY (8 cases), and 45,X/46,XY/47,XYY (6 cases), all other cases were ascertained postnatally. Special emphasis was placed on structural abnormalities. This review includes 11 cases of 46,XYp-; 90 cases of 46,XYq- (52 cases non-mosaic; 38 cases 45,X mosaic); 34 cases of 46,X,r(Y) (9 cases non-mosaic and 25 cases 45,X mosaic); 8 cases of 46,X,i(Yp) (4 non-mosaic and 4 mosaic with 45,X); 12 cases of 46,X,i(Yq) (7 non-mosaic and 5 mosaic); 44 cases of 46,X,idic(Yq); 80 cases of 46,X, idic(Yp) (74 cases had breakpoints at Yq11 and 6 cases had breakpoints at Yq12); 130 cases of Y/autosome translocations (50 cases with a Y/A reciprocal translocation, 20 cases of Y/A translocation in 45,X males, 60 cases of Y/DP or Y/Gp translocations); 52 cases of Y/X translocations [47 cases with der(X); 4 cases with der(Y), and 1 case with 45,X with a der(X)], 7 cases of Y/Y translocations; 151 postnatally diagnosed cases of 45,X/46,XY; 14 postnatally diagnosed cases of 45,X/47,XYY; 18 cases of 45,X/46,XY/47,XYY; and 93 aforementioned prenatally diagnosed cases with a 45,X cell line. It is clear that in the absence of a 45,X cell line, the presence of an entire Yp or a region of it including SRY would lead to a male phenotype in an individual with a Y aneuploidy, whereas the lack of Yp invariably leads to a female phenotype with typical or atypical Ullrich-Turner syndrome (UTS). Once there is a 45,X cell line, regardless of whether there is Yp, Yq, or both Yp and Yq, or even a free Y chromosome in other cell line, there is an increased chance for that individual to be a phenotypic female with UTS manifestations or to have ambiguous external genitalia. This review once again shows a major difference in reported phenotypes between postnatally and prenatally diagnosed cases of 45,X/46,XY, 45,X/47,XYY, and 45,X/46,XY/47,XYY mosaicism. It appears that ascertainment bias can explain the fact that all known patients with postnatal diagnosis are phenotypically abnormal, while over 90% of prenatally diagnosed cases are reported to have a normal male phenotype. Further elucidation of major Y genes and their clinical significance can be expected in the rapidly expanding gene mapping projects. More, consequently better, phenotype/karyotype correlations can be anticipated at both the cytogenetic and the molecular level.