Metformin has failed to gain wide acceptance as a first-line treatment option for women with anovulatory infertility related to polycystic ovary syndrome. This study aimed to ascertain factors that predict fertility success with treatment that included metformin compared to standard (non-metformin) treatment. METHODS: Randomised trial data analysis by logistic regression of factors likely to have a differential influence on the likelihood of success of metformin versus non-metformin treatment amongst women with ovulation dysfunction related to polycystic ovary syndrome. RESULTS: metformin versus those receiving placebo and those with lower BMI who received metformin were more likely to become pregnant than their lower BMI counterparts who received placebo (P=0.039).