Purpose of Investigation: The objective of this prospective controlled observational study was to evaluate the effect of myoinositol supplementation on intracytoplasmic sperm injection (ICSI) outcome in Japanese infertile polycystic ovarian syndrome (PCOS) women with non-obese less-androgenic phenotype, which is common in East Asia. Materials and Methods: The ICSI outcome in the first treatment cycle under a flexible gonadotropin-releasing hormone antagonist protocol was compared between 25 PCOS women taking 4 g/day myoinositol + 400 μg/day folic acid (Myo/FA group) and 25 PCOS women taking 400 μg/day FA alone (FA Group). Results: The total dosage of human menopausal gonadtropin was significantly lower in the Myo/FA group than in the FA group (p = 0.034). The number (p = 0.000029) and rate (relative risk 1.47, 95% confidence interval 1.43-1.50, p < 0.0001) of metaphase II oocytes and the number of fertilized oocytes (p = 0.0074) was significantly higher in the Myo/FA group than in the FA group. Conclusion: Myoinositol supplementation is a safe and effective treatment modality to increase the mature and fertilized oocytes, along with a reduction in the gonadotropin dose in ICSI cycles in Japanese infertile PCOS women with non-obese less-androgenic phenotype.