The effect of seasonal variation in ambient temperature on body weight was studied in a man. Data were collected over three consecutive years in a temperate climate, including a two-week shift in winter to a tropical environment. A strong negative correlation existed between body weight and ambient temperature (r(2) = 0.58, p <0.0001), except for the season with shift to the tropics. In the latter case, body weight dropped from 60.0 to 57.6 kg and remained low until summer, despite ambient temperatures down to -4.7C in late winter. In conclusion, body weight shows a clear seasonal variation triggered by ambient temperature.