Obesity is known to be associated with sub-optimal reproductive performance but its direct effect on the outcome of assisted reproduction techniques (ART) is less clear. This present study aimed to perform a systematic review of the available evidence to assess the effects of obesity on the outcome of ART. A number of observational studies were identified. Interpretation of the results was compromised by variations in the methods used to define overweight and obese populations and inconsistencies in the choice and definition of outcome measures. Compared with women with a BMI of 25 kg/m(2) or less, women with a BMI > or = 25 kg/m(2) have a lower chance of pregnancy following IVF [odds ratio (OR) 0.71, 95% CI: 0.62, 0.81], require higher dose of gonadotrophins (weighed mean differences 210.08, 95% CI: 149.12, 271.05) and have an increased miscarriage rate (OR 1.33, 95% CI: 1.06, 1.68). There is insufficient evidence on the effect of BMI on live birth, cycle cancellation, oocyte recovery and ovarian hyperstimulation syndrome. Further studies with clear entry criteria and uniform reporting of outcomes are needed to investigate the true impact of weight on the outcome of ART.