Health professionals have adopted proactive testing for early evidence of disease. Researchers have identified that this leads to enumerated understandings and shapes behavior in productive ways. Smoking-cessation advisors regularly test clients for carbon monoxide (CO), but client views of this had not previously been explored. We interviewed 23 clients of a United Kingdom-based stop-smoking service regarding their experiences of CO testing. The majority of participants were successful quitters. We used ATLAS.ti 7 as a data-management tool during structured qualitative analysis. Our findings reveal that clients believed the results of their CO tests. Many became enumerated in their understanding, and thus placed themselves in a hierarchy with other members of their group. Almost all clients found that knowing their CO test score was motivating. We conclude that additional research is needed to understand the experiences of CO testing among clients who do not quit.