The biological regulation of appetite is currently an important topic in nutrition, since hyperphagia has been implicated as the prime cause of obesity. Cyclical fluctuations in food intake occur in women across the menstrual cycle, with a periovulatory nadir and a peak in the luteal phase. These alterations in food intake, in response to ovarian steroid hormone changes may be more than 2.5 MJ/day, with the mean reported changes shown in 19 separate studies of 1.0 MJ/day. Hormonal induced fluctuations in food intake could, therefore, contribute to energy imbalance and consequent weight gain. Further, in nutrition studies involving women subjects where the menstrual cycle phase is not controlled, hormonally induced changes in food selection and intake may mask the often considerably smaller changes in response to experimental variables in appetite research.