The purpose of this retrospective study was to compare the frequency of menstrual cyclicity between two groups of patients with polycystic ovary syndrome: women who were followed while on metformin for 3-6 months and those who were followed for >6 months. The results showed that metformin is highly effective in normalizing menstrual cyclicity in women with polycystic ovary syndrome (the overall response rate was 69%, with 88% of responders achieving normal cyclicity), especially with a treatment duration of 6 months or longer (the response rate was 40% higher for women who were treated with metformin for >6 months vs. 3-6 months, 77% vs. 55%).