BACKGROUND: It is commonly asserted that the average American gains 5 lb (2.3 kg) or more over the holiday period between Thanksgiving and New Year's Day, yet few data support this statement. METHODS: To estimate actual holiday-related weight variation, we measured body weight in a convenience sample of 195 adults. The subjects were weighed four times at intervals of six to eight weeks, so that weight change was determined for three periods: preholiday (from late September or early October to mid-November), holiday (from mid-November to early or mid-January), and postholiday (from early or mid-January to late February or early March). A final measurement of body weight was obtained in 165 subjects the following September or October. Data on other vital signs and self-reported health measures were obtained from the patients in order to mask the main outcome of interest. RESULTS: The mean (+/-SD) weight increased significantly during the holiday period (gain, 0.37+/-1.52 kg; P<0.001), but not during the preholiday period (gain, 0.18+/-1.49 kg; P=0.09) or the postholiday period (loss, 0.07+/-1.14 kg; P=0.36). As compared with their weight in late September or early October, the study subjects had an average net weight gain of 0.48+/-2.22 kg in late February or March (P=0.003). Between February or March and the next September or early October, there was no significant additional change in weight (gain, 0.21 kg+/-2.3 kg; P=0.13) for the 165 participants who returned for follow-up. CONCLUSIONS: The average holiday weight gain is less than commonly asserted. Since this gain is not reversed during the spring or summer months, the net 0.48-kg weight gain in the fall and winter probably contributes to the increase in body weight that frequently occurs during adulthood.