We have used bivariate flow karyotype analysis to quantify aberrant X chromosome size in 11 XX males. With one exception, the patients could be grouped into those with an X homologue difference greater than normal (Group A, n = 3) and into those whose X homologue difference could not be distinguished from female controls (Group B, n = 7). The range of sizes of the aberrant X chromosome in Y‐sequence positive patients agrees with the variable nature of the X‐Y interchange in these individuals as determined by the use of Y‐specific DNA probes and Southern blotting analysis. In one patient it was possible to sort separately the normal and the X‐Y interchanged homologues for dot blot analysis. The presence of Y sequences and an increased dose of the zinc finger gene, ZFY, were detected in the X‐Y interchanged homologue. In preliminary studies of 5 male and 6 female controls, it was noted that a consistent difference between the two X homologues in females was found which could not be totally explained by errors of the fitting procedure. We suggest that this difference could be due to X inactivation and that the two X homologues in females might be distinguishable.