A new method for assessing the level of filter vent blocking by smokers has been used, based on image analysis measurements of the mouth end of the butts after smoking. This method has been demonstrated to be highly correlated to the level of vent blocking applied during smoking, and above all to be independent of the smoking regimen. We investigated the incidence and the level of vent blocking among regular smokers of four different commercial brands with filter ventilation of 31, 40, 61 and 70% (tar yields under ISO conditions varying from 9 to 1 mg). We asked more than 32 smokers of each brand (total : 134 smokers) to collect at least 10 of their used butts from cigarettes that they had smoked under natural conditions (not in the laboratory), and we subsequently analysed 1339 butts. More than 62% of the cigarettes had been smoked without any vent blocking, and less than 7.5% of them appeared to have been smoked with complete vent blocking. The percentage of cigarettes smoked which showed some degree of vent blocking increases with the level of filter ventilation. Among the 30% of smoked cigarettes which showed partial vent blocking, the mean level of vent blocking increases with the filter ventilation (from 9.0% for the two lowest to 23.6% for the two highest ventilated brands). These levels of vent blocking have been shown by other authors to have only small effects on the yields. We conclude that the incidence of vent blocking among smokers is small and that when it occurs, it has most often oly small effect on the yields.