The aim of this research project, in which Oxford University together with others participated, was to find relationships between obesity, physical activity, transportation, and CO2-emissions. In Cardiff, Kenilworth, and Southampton, 3643 inhabitants were asked how and how far they travelled. CO2-emissions were calculated based on the responses. They were also questioned about their state of health, their Body Mass Index, and whether physical activity was part of their recreation. Apparently travel behaviour in the week prior to answering the questionnaire caused an average of 18.8 kilograms CO2. Automobiles emitted the most of the CO2 (89.9%), followed by trains (4.4%), and buses (3.8%). There was a clear correlation between excessive body weight and CO2-emission. Analysis of the data revealed that heavier people prefer to travel long distances by motorised vehicle rather than by walking or cycling. Another factor is that heavy people tend to drive heavy cars: 16% of all heavyweights owned very large cars (SUV's, delivery vans, pick-up trucks), as opposed to 10% of people who are not obese. It is clear that walking and cycling in general lead to low CO2-emissions. However, recreational walking and other recreational sports lead to more motorised traffic - hence more CO2-emissions - as people often drive to the recreational venue.