Background The objective of this study was to evaluate the reproducibility and validity of patients’ mouth opening measurements in a research setting. Methods Firstly, 68 patients made repeated self-measurements of mouth opening using a cardboard scale (Therabite Range of Motion Scale – TRMS). Secondly, 80 patients enrolled in a clinical trial on morbidity after lower third molar surgery, made daily assessments during the postoperative week. Patients’ measurements were then compared to gold-standard clinicians’ measurements. Results Reliability of patients’ measurements was excellent with an intraclass correlation coefficient of 0.92. The patient’s measurements correlated well with the gold-standard clinician’s measurements, both for the first 68 patients (Pearson’s rho ranging from 0.86 to 0.90, p < 0.0001) as well as for the 80 patients enrolled in the clinical trial (rho = 0.82, p < 0.0001 at day 2, rho = 0.83, p < 0.0001 at final visit). Conclusions TRMS can be used by patients to produce reproducible and valid mouth opening measurements.