Background and Purpose: To quantify respiratory motion of the vocal cords during normal respiration using 4D-CT. The final goal is to develop a technique for single vocal cord irradiation (SVCI) in early glottic carcinoma. Sparing the non-involved cord and surrounding structures has the potential to preserve voice quality and allow re-irradiation of recurrent and second primary tumors. Material and methods: Four-dimensional CTs of 1 mm slice thickness from 10 early glottic carcinoma patients were acquired. The lateral dimensions of the air gap separating the vocal cords were measured anteriorly, at mid-level and posteriorly at each phase of the 4D-CTs. The corresponding anterior-posterior gaps were similarly measured. Craniocaudal vocal cords movements during breathing were derived from the shifts of the arythenoids. Results: The population-averaged mean gap size the corresponding standard deviation due to breathing (SDB) for the lateral gaps was 5.8 +/- 0.7 mm anteriorly, 8.7 +/- 0.9 mm at mid-level, and 11.0 +/- 1.3 mm posteriorly. Anterior-posterior gap values were 21.7 +/- 0.7 mm, while cranio-caudal shift SDB was 0.8 mm. Conclusion: Vocal cords breathing motions were found to be small relative to their separation. Hence, breathing motion does not seem to be a limiting factor for SVCI. (C) 2008 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 89 (2008) 19-27.