Obesity increases the risk of endometrial and ovarian cancer, and oestrogen receptor (ER)-progesterone receptor (PR)-positive postmenopausal breast cancer. A modest positive association between body mass index (BMI) and cervical cancer has also been found. By contrast, an inverse correlation between BMI and premenopausal breast cancer exists. Endogenous sex hormones, insulin resistance/hyperinsulinaemia, adipokines, cytokines and chronic inflammation, among other factors, may be involved in the promotion of cancer in obese patients. Obesity is also associated with an increased risk of cancer recurrence and mortality most likely due to suboptimal treatment and/or co-morbidities. It is recommended that chemotherapy doses be calculated on the actual body weight and that radical surgery be performed as in non-obese patients. The high risk of peri-operative complications may be reduced by optimizing preoperative clinical conditions. As part of cancer prevention, obese women should be encouraged to adopt healthy lifestyles leading to weight loss and to undergo regular cancer screening.