Transnasal flexible laryngoscopy (TFL) is an examination of laryngeal anatomy and physiology using continuous light. TFL is being used increasingly by voice pathologists in treatment but with little scientific evidence to support it. The purpose of this study was to evaluate the effectiveness and efficiency of TFL as a therapeutic tool. The study used a prospective randomized controlled trial. Fifty dysphonic subjects were recruited and randomly assigned to either a traditional treatment group or a TFL-assisted treatment group. The effectiveness of voice therapy in both treatment groups was measured with a package of voice outcome measures. Subjects in both treatment groups demonstrated statistically significant improvements after voice therapy (perceptual auditory rating of voice quality measurement p < 0.01; instrumental electroglottographic measurement p < 0.01; patient questionnaire measurement p < 0.01). The time taken to complete treatment in both groups was recorded. The average (median) time taken to complete voice therapy in the TFL-assisted treatment group was 2 hours less than in the traditional treatment group (p < 0.01). Voice therapy with TFL as a therapy tool was effective and more efficient than traditional voice therapy.