- Aims The aim of this work was to study the association between obesity and smoking habits in young adults. Specifically, we tested the hypothesis that obesity does not prevent young adults from smoking and conversely smoking does not protect against obesity. Design and setting Trained nurses interviewed participants concerning demographic data and health behaviors such as smoking. At the time of the interview, weight and height were measured. Data were analyzed retrospectively. Participants A representative sample of Israel Defense Force (IDF) personnel upon discharge from compulsory service, usually at the age of 20–21 years. Findings Overall, 29 745 participants were included during the 13-year study (16 363 males and 13 382 females). Smoking rates were higher among obese participants than among overweight and non-obese participants (34.9%, 37.1%, 43.6% for non-obese, overweight and obese, respectively; P < 0.001). Mean number of cigarettes smoked per day were also higher among smokers that were obese and overweight compared to the non-obese (15.2 ± 9.2, 15.6 ± 10.7, 18.0 ± 9.8, respectively; P < 0.001). Overweight and obesity were associated with the father's lower academic educational level. In logistic regression analysis, obesity, year of study and parental academic education were correlated independently with smoking (P < 0.001). Conclusion The positive association between obesity and smoking suggests that obesity is not a deterrent to smoking and also that smoking does not help to prevent obesity.