Aromatase inhibitors (AIs) decrease the production of oestrogen, decreasing stimulation of hormone receptor-positive breast cancer. Theoretically, AIs may be less effective in obese women, due to the greater quantity of aromatase in peripheral fatty tissue. We performed a systematic review to assess the effect of obesity on AI efficacy in breast cancer treatment. The review followed PRISMA guidelines. Studies included were interventional or observational studies with comparison groups, of postmenopausal women with hormone receptor-positive breast cancer on treatment with an AI, alone or in combination with other drugs, in which body mass index or another measure of obesity was recorded. Studies in all languages were included; if published as an abstract only, authors were contacted for further information. Outcome measures included overall survival, disease-free survival or time to progressive disease, survival from the start of therapy, mortality measures, local or distant recurrence of primary cancer and time to recurrence. Of 2,344 citations identified from five databases, eight studies met the criteria for inclusion; three randomised controlled trials and five retrospective cohort studies. Due to variability in study factors, it was not possible to perform a quantitative meta-analysis. However, the systematic review showed a trend towards a negative effect of obesity on AI efficacy. There is evidence of a negative effect of obesity on AI efficacy in postmenopausal hormone receptor-positive breast cancer, but the size of the effect cannot be assessed. More information is needed before clinical recommendations are made.