In some countries, the incidence of obesity doubles every 10 years. For the obstetrician-gynecologist, there are many different situations where the patient's excess body weight calls for an adapted diagnostic and therapeutic approach. Obesity does not in itself appear to be a factor lowering fertility. However obesity-induced hormone disorders could contribute, in certain cases, to biological imbalance and thus favor the development of ovulation dysfunction. Pregnancy in obese women should be managed as a high risk pregnancy. The incidence of gestational diabetes and hypertension is increased. Macrosomatia is frequent. There is a 2- to 3-fold increase in the rate of cesarean sections with more complications. Fetal morbidity does not appear to be changed when maternal weight gain is limited. With obesity, there is an increased risk for breast and endometrial cancer due, for most authors, to elevated levels of circulating estrogens resulting from aromatization of male sex steroids in adipose tissue and decreased levels of sex hormone-binding globulin. Anesthesia and surgery in obese patients can be problematic and special care must be taken to prevent further morbidity. Laparoscopic surgery is possible under certain conditions, although its role remains to be determined. Prescription of hormone replacement must take into consideration several parameters which determine its usefulness and surveillance. Obesity is not a contraindication for hormone replacement therapy but is frequently a non-indication.