Two studies were conducted to examine the effect of filter vent blocking and smoking topography on carbon monoxide (CO) levels in smokers. In Study 1, 12 participants smoked two types of cigarettes (Marlboro Light and Carlton 100) under two types of blocking conditions (unblocked and half-blocked) while using a smoking topography device. Participants were restricted to 8 puffs, separated by 45 s. Significant main effects of CO boost for cigarette type and blocking condition replicated previous findings. A significant increase in CO boost for the Marlboro Light blocked condition is a novel finding for this best-selling brand. That result and the finding that topography measures did not predict CO boost made us question the reliability of CO boost. In Study 2, we examined the reliability of CO boost by recruiting 12 participants to smoke three unblocked Carlton 100 cigarettes in one session and three half-blocked in another. CO boost was significantly greater for the blocked sessions compared to the unblocked and CO boost did not differ within session, thus supporting the reliability of the measure. When participants do not switch brands within a session, smoking topography measures are predictive of CO boost.