Introduction and objectives: Frequent consumption of commercially-prepared meals appears to be associated with obesity. Such a relationship could also be observed between home cooking practices and obesity but it has not been documented so far. We investigated the prospective association between food preparation behaviours and 5-year weight gain and the onset of obesity after 5 years in adults. Method: We prospectively examine 5-year weight gain and the risk of obesity after 5 years in 3,629 men and 9,221 women aged ≥ 18 years participating in the NutriNet-Santé cohort study. Self-reported weight, height and cooking practices were collected at baseline and 5 years later. Food preparation behaviours were assessed at baseline using a web-based questionnaire. Cooking skills, preparation from scratch, i.e. use of raw or fresh ingredients that had no or minimal processing and kitchen equipment were assessed using a 0-10-point score and, frequency of meal preparation, enjoyment of cooking and willingness to cook better or more frequently were categorical variables. Associations between various dimensions of food preparation behaviours and 5 years weight gain and risk to be obese 5 years later were respectively assessed using analysis of covariance and logistic regression models stratified by sex and adjusted for age, household composition and socio-economic factors. Results: In both genders, associations of frequency of meal preparation, enjoyment of cooking and willingness to cook better or more frequently, cooking skills and kitchen equipment with 5-year weight gain or obesity risk were not statistically significant (P>0.05). In women, score of preparation from scratch was inversely associated with risk to be obese five years later (OR=0.92 (0.86; 0.98), P=0.03) and women with low score of preparation from scratch more gained weight over 5 years (mean weight gain=1.69 (0.14)) compared with those with the highest score (mean weigh gain=0.87 (0.11), P=0.04). Conclusion: Use of raw or fresh ingredients that had no or minimal processing was related to lower risks of obesity and weight gain in women. Such complex relationships should be taken into account to effectively target public health efforts in obesity prevention.