Objective. The association between smoking and idiopathic inflammatory bowel disease is well known; smoking seems to have a diverse effect. Crohn's disease is associated with smoking, while ulcerative colitis is associated with non-smoking. Data on smoking in microscopic colitis of the collagenous type (CC) are lacking. The aim of this investigation was to study smoking habits in CC and to observe whether smoking had any impact on the course of the disease. Materials and methods. 116 patients (92 women) with median age of 62 years (interquartile range 55-73) answered questionnaires covering demographic data, smoking habits and disease activity. As control group we used data from the general population in Sweden retrieved from Statistics Sweden, the central bureau for national socioeconomic information. Results. Of the 116 CC patients, 37% were smokers compared with 17% of controls (p < 0.001, odds ratio (OR) 2.95). In the age group 16-44 years, 75% of CC patients were smokers compared with 15% of controls (p < 0.001, OR 16.54). All CC smoker patients started smoking before the onset of disease. Furthermore, smokers developed the disease earlier than non-smokers - at 42 years of age (median) compared with 56 years in non-smokers (p < 0.003). Although the proportion with active disease did not differ between smokers and non-smokers, there was a trend indicating that more smokers received active treatment (42% vs. 17%, p = 0.078). Conclusions. Smoking is a risk factor for CC. Smokers develop their disease more than 10 years earlier than non-smokers.