OBJECTIVES This study was conducted to determine the usefulness of metformin therapy in improving outcomes related to infertility in patients with polycystic ovary syndrome (PCOS). A Bayesian meta-analytic and mixed treatment comparison (MTC) approach was used. METHODS An electronic literature search was performed using PubMed and the Cochrane Central Register of Controlled Trials to identify randomized controlled trials that reported at least one of the outcomes of interest – ovulation, pregnancy and live birth in PCOS patients randomized to treatment with either metformin, clomiphene citrate (CC) or combination of these drugs, which included a comparison with either placebo or each other. Reference lists of meta-analyses and reviews were hand searched to identify any additional articles. Bayesian meta-analyses were conducted for each outcome separately and for different therapeutic comparisons with metformin. Additionally, Bayesian MTCs were also conducted for each outcome. Analyses were performed using random effects models. RESULTS A total of 27 RCTs were identified and 24 studies reported outcomes in a usable form for inclusion in the analysis. The total number of patients was 2217. The meta-analyses revealed that metformin was superior to placebo for ovulation induction (median OR=2.9 with 95%[CrI] 1.6–6.0). Comparison of metformin and CC to CC alone revealed that combination therapy was superior in both ovulation induction (median OR=4.2 with 95%[CrI] 1.5–12.3) and pregnancy (median OR=5.0 with 95%[CrI] 1.7–22.4). When live birth was considered there was no significant difference between combination therapy and CC alone (median OR=2.2 with 95%[CrI] 0.4–55.5). In the MTC, the efficacy of the therapeutic comparisons for ovulation and pregnancy in descending ranking order was combination therapy, CC alone, metformin alone and placebo. CONCLUSIONS Combination therapy with metformin and CC is more effective than CC alone in ovulation and pregnancy outcomes in women with PCOS.