Helplines are increasingly used to provide information and support for people affected by cancer, and the distress routinely associated with diagnosis and treatment is a major focus for those providing such care. Little is known, however, about how the Distress Thermometer (DT), a widely used tool for the assessment of patient/carer distress on cancer-support telephone helplines, is introduced and used in such settings. Using the method of conversation analysis, we present a qualitative analysis of DT use in actual telephone interactions by looking closely at how particular practices shape interaction on a cancer helpline. Specifically, we examine how oncology-trained nurse call-takers used the DT, in situ, as a tool for assessing callers, as well as examining how callers responded to this brief screening tool. Our findings show how particular positioning of the DT in the call, and particular forms of its delivery, tend to generate brief responses from callers that avoid topicalization of distress, and tend not to be associated with referral to support services. Implications for successful integration of the DT as a screening tool in cancer- and other health-helpline interactions, as well as for effective training of users, are discussed.