This paper describes two studies that illustrate the utility of listener ratings of speech naturalness for measuring and modifying speech naturalness during a stuttering therapy program. The program involved 5 adolescent stutterers who were receiving an intensive treatment incorporating a prolonged speech procedure. In Study A, a clinician used a 9-point rating scale to score the speech naturalness of 1-min speaking samples each stutter made at intervals over the course of the program. The results demonstrated predictable trends in speech naturalness during the program, but they also showed that natural sounding speech is not a predictable outcome of a procedure that removes stuttering, controls speaking rate, and exposed clients to transfer procedures. In Study B, 3 of the 5 stutterers participated in single subject experiments partway through their therapy program. These experiments were designed to assess the effect of regular feedback of speech naturalness ratings on the stutterer's spontaneous speech. The results showed that each subject's speech naturalness ratings could be modified toward a target level of speech naturalness.