OBJECTIVES The aim of this study was to investigate the development of radiotherapy (RT)-induced mucosal thickening (MT) of the contralateral sinuses in patients with nasal cavity and/or paranasal sinus carcinoma. METHODS We retrospectively reviewed the medical records and the initial and follow-up computed tomography (CT) scans of 37 patients with RT and 10 controls without RT. The CT scans were scored on the Lund-Mackay (LM) staging system. RESULTS Fifteen of the 37 patients had MT before RT, and the mean LM score was 0.68. The MT incidence significantly increased, to 72.9% (p = 0.009), and the LM score significantly increased, to 2.84 (p < 0.001), by 3 months after RT, after which the LM score decreased gradually to 1.73 at 36 months after RT. Four of the 10 controls had MT before treatment, and their mean LM score was 0.7. Their MT incidence and LM score had not changed significantly at 3 months after treatment. The pretreatment LM scores of the patient group and the control group were not significantly different, but their posttreatment LM scores were significantly different at the 3-month follow-up (p = 0.033). CONCLUSIONS Use of RT in patients with nasal cavity and/or paranasal sinus carcinoma may cause a significant increase in the incidence of MT and in the LM scores in all sites of the paranasal sinuses by 3 months after RT, after which the LM score decreases gradually.