A previously published study reporting the use of a modified swim-up technique for sperm preparation prior to insemination which resulted in a high percentage of male births has been criticized for its lack of controls. The present prospective study was initiated to investigate further the efficacy of modified swim-up preparation for male sex-selection when applied in a properly defined control group. Our results showed that the proportion of males born in singleton pregnancies was approximately 50% in the group inseminated following sperm preparation with Percoll and in the control group with no sperm preparation compared with 88.5% in the group treated with the modified method of swim-up sperm preparation prior to insemination. This high rate of males in the group treated with modified swim-up was also observed in singleton pregnancies of women taking ovulation inducing drugs (primarily clomiphene citrate). This contrasts with previous publications in which a higher rate of females was found in clomiphene citrate patients using the albumin separation technique. How the mechanism of the swim-up procedure may result in a high male birth rate remains unclear. A high percentage of y enriched semen was found using fluorochrome quinacrine mustard staining but this procedure may falsely stain autosomal chromosomes. If analysis using sensitive DNA probes fails to confirm the y enrichment of the spermatozoa, one must hypothesize that the modified swim-up procedure damages the x-spermatozoa.