Organisms acquire more calories from eating hot food than eating the identical food cold; thus, the widespread use of microwave ovens might have played a small role in the current obesity epidemic, just as the widespread use of refrigerators might have retarded the historic increase in obesity a century ago. Analysis of the British Cohort Study showed that, net of dietary habit, physical activities, genetic predisposition, and other demographic factors, the ownership of a microwave was associated with an increase of .781 in body mass index (BMI) and 2.1 kg in weight (when the ownership of other kitchen appliances was not associated with increased BMI or weight), and it more than doubled the odds of being overweight. In the United States from 1960 to 2015, the adult overweight, obesity, and extreme obesity rates were very highly correlated (r= .94–.98) with the proportion of households with microwaves, and it was not because both were consequences of increasing wealth. Net of median household income, the proportion of households with microwaves was very strongly (ds > 1.0) associated with adult overweight, obesity, and extreme obesity rates, while median household income was not at all associated with them. Individual data from the United Kingdom and historical data from the United States highlighted the possible role of the widespread use of microwave ovens in the obesity epidemic.