Twenty two normal subjects and 13 maturity onset diabetic patients completed an observational study with repeated weighing and fasting blood tests from one month before to one month after Christmas. Over the Christmas period in all subjects an increase in weight was observed (mean 0.8 (SD 0.1) kg, p less than 0.001), which was maintained through January. This may signify an average additional 6000 kcal ingested. By three to six days after Christmas a slight but significant increase in fasting plasma triglyceride (p less than 0.03) and cholesterol (p less than 0.02) concentrations occurred, with a subsequent rise in glycosylated haemoglobin concentration (p less than 0.001). This study is not likely to affect any future Christmas.