BACKGROUND: Prophylactic cranial irradiation (PCI) has been considered standard treatment in patients with limited-stage small cell lung cancer(LS-SCLC) in complete remission following systemic therapy. But little is known about whether delay in delivering PCI after initiation of primary chemotherapy will lead to a detrimental outcome. METHODS: From March 2005 to December 2010, 479 patients with LS-SCLC were treated in Zhejiang Cancer Hospital, with 399 patients deemed eligible. All patients received ≥ 3 cycles of cisplatin-based chemotherapy and 344 patients received thoracic radiotherapy . PCI was administered to 185 patients. 92 patients had "early" PCI (administered within 6 months after start of primary chemotherapy)and 93 had "late" PCI (administered 6 months later). RESULTS: The cumulative risks of symptomatic brain metastases at 6, 12 and 24 months were 7％ ,29% and 42% respectively, in no PCI group and 0, 3% and 13% in PCI group(P=0.000). The hazard ratio(HR) for the PCI group was 0.24 (95% confidence interval [CI], 0.15 to 0.39).However, No significant difference was found between "early" PCI group and "late" PCI group (P =0.875).For the entire group, the median survival was 27.5 months, and overall survival (OS) rates at 1-,3-year were 89% and 43%, respectively. The median survival was 38.8 months in PCI group and 21.5 months in no PCI group (P =0.000;HR,0.60;95%CI:0.45~0.79) . The median survival was 32.6 months in "early" PCI group and 40.9 months in "late" PCI group (P=0.361) . Multivariate analysis revealed that PCI (P = 0.004) and thoracic radiotherapy (P=0.023) were the only 2 independent favorable prognostic factors of OS.CONCLUSIONS: PCI significantly decreased the incidence of brain metastases and improved overall survival in patients with LS- SCLC. "Early" PCI was as effective as "late" PCI.