Factors which effect the expired air carbon monoxide (CO) levels of smokers were examined in matched subject pairs who smoked an equal number of daytime cigarettes but had different CO levels (mean difference = 15.4 ppm). Measures of puff number, duration, and spacing, as well as the amount of CO increase per cigarette (CO boost), were assessed while subjects smoked a single cigarette in daily laboratory sessions. Subjects with relatively high CO levels had larger increases in CO after smoking a single cigarette than did individuals with low CO levels (p. less than .005) but did not differ on any other smoking topography measure. These data suggest that simple topography measures of puff number and duration may not contribute to between subject differences in tobacco smoke exposure, and that greater attention should be given to more refined measures such as puff volume and depth of inhalation. These data also suggest that the measurement of CO boost per cigarette may provide useful information regarding tobacco smoke intake.