||Vocal hyperfunction, characterized by excessive laryngeal muscle tension, is a condition associated with numerous voice disorders, including primary muscle tension dysphonia (pMTD). Primary MTD is disturbance occurring in the absence of structural or neurologic pathology. Vocal hyperfunction is postulated to be the proximal cause of the dysphonia in pMTD. A measure of relative fundamental frequency (RFF) has been proposed as an objective and noninvasive marker of vocal hyperfunction. This retrospective study examined the clinical utility of RFF in identifying and tracking changes in vocal hyperfunction before and following a course of manual circumlaryngeal therapy for pMTD patients. Because RFF is a time-based acoustic measure reliant on periodicity for accurate calculation, additional investigation into the influence of dysphonia severity on its validity across the severity spectrum was completed. RFF calculations were derived from pre- and posttreatment audio recordings from 111 females with pMTD and 20 vocally normal controls. Three voiced-voiceless stimuli (VCV tokens) were analyzed. Listener ratings of dysphonia severity were employed to determine (1) the relation of RFF measures to overall severity, and (2) the effects of dysphonia severity on the utility of RFF calculations. Multiple regression analyses demonstrated that RFF onset slope consistently varied as a function of group membership and therapy time conditions. Pearson Product-Moment Correlations showed a significant relationship between RFF onset cycle 1 values and listener ratings of dysphonia severity. In addition, regression analysis confirmed the influence of therapy condition and specific RFF cycles on dysphonia severity rating. Cumulatively, the analyses confirmed that RFF onset may be sensitive to predicting the presence and degree of vocal hyperfunction before and after therapy, and as an index of dysphonia severity. However, RFF could not be fully analyzed in many subjects, yielding a large quantity of missing data. Adjusted odds ratio estimates revealed that these unanalyzable data were related to phonetic context (token), group membership (pMTD vs. control) and severity level of dysphonia. Although RFF showed potential as an objective measure of vocal hyperfunction before and following voice therapy, the large number of unanalyzable samples (related to increased dysphonia severity especially in the pMTD group) represents a critical limitation.