The effect of Christmas time on body weight development was measured in 46 obese patients in maintenance therapy at the Obesity Unit of the Karolinska Hospital and in 76 hospital staff controls of similar sex and age distribution. In controls a statistically significant weight increase of 0.4 +/- 0.8 (s.d.) kg (P < 0.001) was found, whereas the mean increase in the obese patients of 0.6 +/- 2.4 kg was not statistically significant. Little variation in weight development over Christmas was found in controls, whereas the weight change in the obese group ranged from +6.1 to -8.8 kg over the 2-3 weeks. Seasonal variation in body weight has been attributed to circadian rhythms. Cultural food habits may also play a role.