Background: Previous studies reported that patients with preexisting radiological interstitial lung abnormalities (ILAs) were more susceptible to developing radiation pneumonitis (RP) after thoracic radiation therapy (TRT). The present study aimed to evaluate the incidence and predictors of RP after TRT in patients with small-cell lung cancer (SCLC) with or without preexisting radiological ILAs. Methods: A total of 95 consecutive patients with SCLC between January 2015 and December 2015, who were treated with thoracic intensity-modulated radiation therapy at Shanghai Pulmonary Hospital,Tongji University School of Medicine, were analyzed. The diagnosis of ILAs was reviewed by two experienced thoracic radiologists based on the pretreatment high-resolution computed tomography imaging, such as honeycombing, subpleural reticular opacities, ground-glass opacity, and traction bronchiectasis. Univariate and multivariate analyses were used to assess the correlation of clinical factors, preexisting radiological ILAs, and dose-volume histogram-based dosimetric parameters with RP. Results: Fifteen (15.8%) patients had preexisting radiological ILAs. The incidence of >= grade 2 and 3 RP at 1 year was 27.1% and 12.7% in the entire cohort, respectively. Preexisting radiological ILAs were associated with an increased risk of >= grade 2 RP (50.0% in ILAs + vs 23.3% in ILAs-, P = 0.017) and >= grade 3 RP (35.8% in ILAs + vs 8.9% in ILAs-, P= 0.005) at 1 year. Preexisting radiological ILAs and smoking history (>= 40 pack-years of smoking) were significant predictors of >= grade 3 RP in multivariate analysis (P= 0.023 and 0.012, respectively). Conclusions: Preexisting radiological ILAs and smoking history (>= 40 pack-years of smoking) are associated with an increased risk of >= grade 3 RP after TRT in patients with SCLC.