BACKGROUND: The glycoprotein YKL-40 is synthesized both by cancer cells and by tumor-associated macrophages and plays a functional role in tumor progression. Consequently, high serum YKL-40 levels have been associated with a poor prognosis in patients with several cancer types. However, the role of YKL-40 has not been established in non-small cell lung cancer (NSCLC). METHODS: Pretreatment serum levels of YKL-40 were determined in 189 patients with NSCLC (143 men and 46 women; median age, 62 years;, age range, 41-76 years). Twelve percent of patients had stage IIIB disease, and 88% had stage IV disease. Ninety-eight patients received combined gemcitabine and vinorelbine, and 91 received combined gemcitabine, vinorelbine, and cisplatin as first-line chemotherapy. The median overall survival was 37 weeks. RESULTS: Patients had a median serum YKL-40 level of 209 ng/mL (range, 19-2153 ng/mL). No correlation was observed between overall survival and the type of chemotherapy regimen used, tumor stage, sex, or histologic types. Patients with high serum YKL-40 levels (greater than the median level for all patients [209 ng/mL]) had a significantly shorter survival than patients with serum YKL-40 levels below the median (median survival, 32 weeks vs 41 weeks; P = .007). In multivariate analysis, the serum YKL-40 level, the presence of bone lesions, and the serum lactate dehydrogenase level were independent, statistically significant prognostic factors. CONCLUSIONS: The pretreatment serum YKL-40 level was identified as a new, independent prognostic biomarker in patients with metastatic NSCLC and may help to determine the individual prognosis of these patients. Cancer 2010;116:4114-21. (C) 2010 American Cancer Society