The Patient’ s providing an extended answer to the doctor’ s yes / no interrogative is frequently observed in medical interaction. Though an extended answer violates the grammatical constraint from a yes / no interrogative, and takes more time in clinical interaction , the patient’ s extended answers are not always negative. This paper, adopting conversation analysis as its research methodology and taking naturally occurring doctor-patient interaction at urology department as its data, probes into the sequential environments and reasons for the patient’ s extended answer construction, furthermore, the value and function of extended answers has also been summarized. Data analyses support that the doctor could permit the patient to take more time to construct extended answers in clinical interaction, because the information in extended answers might have certain medical value. The doctor’s permitting the patient to construct extended answers not only help the doctor to carry out effective diagnosis and treatment, but also to build harmonious doctor-patient relationship.