F ecal volatile organic compounds (VOCs) are gaseous carbon-based metabolic products that may serve as diagnostic biomarkers for gastrointestinal diseases, including advanced adenomas, colorectal carcinoma, and inflammatory bowel disease. 1,2 Fecal VOCs are considered to reflect intestinal microbiota composition, which is influenced by external factors including diet, medication, and smoking. 3,4 However, data on the influence of smoking on VOC composition are not yet available. Our aim was to assess the effect of smoking on fecal VOC composition. Methods Study Design Adult patients scheduled for colonoscopy in the VU University Medical Center (Amsterdam, The Netherlands) were eligible to participate in this study, only patients without endoscopic abnormalities (including polyps, inflammatory bowel disease, diverticula, or colorectal cancer) and antibiotic use in the preceding 3 months were included. All patients were enrolled consecutively between February 2015 and November 2016. Fecal samples were collected at home by the participants in a sterile container before bowel cleansing and subsequent colonoscopy and stored at-20 C until analysis by electronic nose (Cyranose 320; Smiths Detection, Pasadena, CA). The ambient air in the lavatories was not analyzed. Standard demographics and smoking habits were collected by means of a questionnaire. Fecal VOC profiles from current smokers, former smokers, and nonsmokers were compared. This study was approved by the medical ethical committee of the VU University Medical Center. Results Patient Population Fecal samples from 56 subjects (11 smokers, 21 nonsmokers, and 24 former smokers) were analyzed. The median age was 62 years (range, 27-82 y), with a male to female ratio of 1:1. There were no statistically significant differences in groups for the variables of age, sex, body mass index, diet, sample weight, chronic diseases, and medication and supplement use. Volatile Organic Compounds Analysis There was a statistically significant difference between fecal VOC patterns for all groups (smoker,