BACKGROUND: Non-smoking status has consistently been reported to be an independent predictive risk factor of PONV. At first view, this empirical observation is interesting. However, there are two unresolved issues. Firstly, it remains unclear, why non-smoking patients are at a increased risk of PONV, and consequently, smoking patients are "protected". For instance, the investigators failed to show any benefit of (short-term) perioperative transcutaneous nicotine administration for the prevention of PONV. Secondly, no PONV risk score has shown satisfactory sensitivity and specificity, largely limiting their clinical applicability. It has been argued that one weakness of these scores was that non-smoking status was ill defined and was treated in a purely dichotomous fashion. For instance, it remains unclear from what length of abstinence the formerly PONV-protective effect will be lost in an ex-smokers. Also, it remains unknown whether "heavy" smokers" are more protected. Finally, there are various ways to consume tobacco; tobacco snuffing was also shown to decrease the risk of PONV. AIMS: 1. To test whether the risk of PONV differs between non-, ex- and current smokers. 2. To test whether there is a dose-effect relationship between the amount of tobacco smoking, and the risk of PONV. 3. To test in ex-smokers for a relationship between the duration of smoking cessation and the risk of PONV. METHODS: Patients who participate in the main study will be asked, pre-operatively, to fill in a questionnaire regarding smoking habits. Patients will be classified into non-smokers (exposed or not to passive smoking), ex-smokers (stop >1 month; exposed or not to passive smoking; substituted), and smokers. Variables related to PONV and to smoking (potential confounders) will be recorded.