Small cell lung cancer (SCLC) is characterized by a high propensity for metastases and a poor prognosis irrespective of high sensitivity for initial chemotherapy. Although interstitial pneumonia (IP) is one of risk factors for lung cancer, efficacy of cytotoxic chemotherapy for patients with SCLC with IP remains unclear. Our study aims to evaluate the efficacy of systemic chemotherapy and assess risk of acute exacerbation (AE)-IP with cytotoxic drugs for extensive disease (ED)-SCLC patients with IP. We performed a retrospective study of 192 consecutive ED-SCLC patients with IP (n = 40) and without IP (n = 152) between 2008 and 2016. 31 of 40 ED-SCLC patients with IP and 130 of 152 patients without IP received systemic chemotherapy. The efficacy of chemotherapy in patients with IP was not inferior to that in patients without IP (overall survival OS, 7.1 95% confidence interval (CI): 0.2-14.0 vs. 10.0 95% CI: 8.2-11.8 months, P = 0.57). Pretreatment serum levels of lactate dehydrogenase (LDH; 651.7 ± 481.0 vs. 301.4 ± 110.7 U/mL, P = 0.01) and C-reactive protein (CRP; 8.9 ± 9.6 vs. 1.8 ± 1.8 U/mL, P = 0.008) were correlated with developed AE-IP in the ED-SCLC patients with IP. Systemic chemotherapy was effective even in ED-SCLC patients with IP. However, the risk of developed AE-IP that was high in patients with IP and should be evaluated using serum LDH and CRP levels before initial chemotherapy.