Background: Primary tumor size is a known prognostic factor for patients with early stage NSCLC treated with either surgery or radiation therapy. Although tumor volume has been associated with outcomes in patients with metastatic disease, it is labor intensive and rarely reported outside of a clinical trial. Since the primary tumor size is more commonly described, we evaluated its prognostic impact in patients with metastatic disease. Methods: The SEER was searched for patients with stage M1b NSCLC, with known tumor (T), lymph node (N) status, and diagnosed between 2004 and 2008. Patients with T0 and malignant pleural effusion were excluded. Tumor size is reported as the largest diameter and was subdivided into S1 (0.1-3 cm), S2 (3.1-5 cm), S3 (5.1-7 cm) and S4 (7.1-20 cm), roughly corresponding to T1, T2a, T2b and T3. Overall survival (OS) was estimated by the Kaplan-Meier method, while the hazard ratios (HR) were estimated and compared by Cox proportional hazard models. Results: Tumor size was available in 21879 (84.4%) out of 25919 patients with complete TNM staging. The frequencies of S1, S2, S3 and S4 were 33.4%, 33.8%, 17.1% and 13.7% respectively. 1-year OS rates for S1 to S4 were 34%, 27.9%, 24.0% and 19.0% respectively. Primary tumor size was an independent predictor for OS after adjustment for age, gender, race, histology, T and N status (p < 0.0001). The decreased OS from each subsequent category of tumor size was statistically significant in both univariate and multivariable analyses (Table). Conclusions: Primary tumor size is readily available and represents a significant prognostic factor for survival in patients with stage M1b NSCLC, independently of T and N status.