6562 Background: Ras is a GDP/GTP binding G protein that acts as a molecular switch converting signals from the cell membrane to the nucleus to regulate cell proliferation, differentiation, and protein synthesis. Activation of ras oncogenes has been identified in a variety of cancers, including 30% of acute myelogenous leukemia patients. The purpose of our study was to evaluate serum ras levels and correlate with survival in hematologic cancer patients. Methods: A novel ras p21 ELISA (Oncogene Science/Bayer Diagnostics, Cambridge, MA) employing two monoclonal antibodies reactive with H, K, and N ras was utilized to quantify total ras levels in serum obtained from patients with various hematologic malignancies including acute myelogenous leukemia (AML), acute lymphocytic leukemia (ALL), chronic myelogenous leukemia (CML), and chronic lymphocytic leukemia (CLL). Results: The total leukemia patient group consisted of 52 patients. At the 75th percentile serum ras cutpoint (524 pg/ml) 11/52 patients were defined as elevated for serum ras. From this patient group, 38 patients had clinical followup available and were included in the Kaplan-Meier analysis of overall survival. Patients with elevated serum ras (>524 pg/ml) had significantly shorter overall survival compared to those without (median OS 205 vs. 677 days) (p= 0.04). In a multivariate analysis including serum ras level and type of leukemia, serum ras level remained a significant independent variable for shorter overall survival (p=0.004). Within leukemia subtypes 2/18 AML, 4/9 CML, 3/7 ALL, and 0/4 CLL patients had elevated serum ras levels. Conclusions: Leukemia patients with elevated serum ras levels had a significantly shorter overall survival. Serum ras should be evaluated as a potential biomarker in larger leukemia trials, especially for response to treatment with inhibitors of the ras signaling pathway. [Table: see text]