To evaluate the toxicity and efficacy of thoracic stereotactic body radiotherapy (SBRT) in patients with subclinical interstitial lung disease (ILD). One hundred patients with 124 lung tumors were treated with SBRT at our institution according to our own protocols; patients with subclinical (untreated and oxygen-free) ILD were treated with SBRT, while those with clinical ILD (post- or under treatment) were not. The administration of 48 Gy in four fractions was used in 103 (83%) of the 124 tumors. The presence of subclinical ILD in the pre-SBRT CT findings was reviewed by two chest radiologists. The relationships between radiation pneumonitis (RP) and clinical factors were investigated. Subclinical ILD was recognized in 16 (16%) of 100 patients. Grade 2-5 RP was recognized in 13 (13%) of 100 patients. Grade 2-5 RP was observed in three (19%) of 16 patients with subclinical ILD. Subclinical ILD was not found to be a significant factor influencing Grade 2-5 RP; however, extensive RP beyond the irradiated field, including the contralateral lung, was recognized in only three patients with subclinical ILD, and the rate of extensive RP was significantly high in the patients with subclinical ILD. Grade 4 or 5 extensive RP was recognized in only two patients with subclinical ILD. Dosimetric factors of the lungs (V5, V10, V15, V20, V25, MLD) were significantly associated with Grade 2-5 RP. The three-year overall survival and local control rates of all patients were 53% and 86%, respectively. No significant differences were seen in either overall survival or local control rates between the patients with ILD and those without ILD. Subclinical ILD was not found to be a significant factor for Grade 2-5 RP or clinical outcomes in the current study; however, uncommon extensive RP can occur in patients with subclinical ILD. Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.