Objective: To evaluate the success for gender selection using a sample of semen separated by a modified swim-up technique. Design: We retrospectively compared the gender outcome of two treatments (A and B) for either a male or female offspring with those who conceived spontaneously. Setting: Private practice of one author (M.A.K.). Patients, Participants: The treatment groups consisted of 52 total pregnancies for couples who conceived by the separation technique. Of these 52 participants, 15 desired a female offspring and were placed into treatment A and 37 desired a male offspring and were placed into treatment B. The control groups consisted of 162 women who were presented with initial consultation for gender selection and conceived spontaneously. Control group A consisted of 80 women who initially chose a female offspring, and control group B consisted of 82 participants who initially chose a male. Interventions: In treatment group A, one timed intrauterine insemination (IUI) was carried out with the bottom 0.5 ml of the separated semen on cycle days 12–14, when the follicle was 18–22 mm. Patients in this group were also administered clomiphene citrate and human chorionic gonadotropin. In treatment group B, one timed IUI was done with the top 0.5 ml of the separated semen, when the follicle was 18–22 mm. Main Outcome Measure: The gender outcome of the pregnancies of two treatment and control groups was evaluated based on the known desired gender. Results: The success rate for conceiving a female child after intervention (treatment group A) was 86.7% effective (p = 0.002) as compared to the control group A. Couples seeking a male child (treatment group B) were 89.2% effective (p = 0.0002) as compared to the control group B. Conclusions: This study reveals that the modified swim-up method with additional monitoring results in statistically significant gender preselection.