In an effort to identify a clinical biomarker for lung cancer, we used cDNA microarray and 2D protein analyses to demonstrate that increased Fas-associated death domain (FADD) mRNA and protein were significantly associated with poor survival. Analyses of copy number and sequence of the FADD gene in 24 independent tumors ruled out the existence of an amplified and/or mutated FADD gene in aggressive lung cancers. Immunohistochemistry-based tissue microarray analysis showed that nuclear localization of FADD and elevation of the phosphorylated form of FADD (p-FADD) correlated with poor outcome (P = 0.003). Tumors with increased p-FADD expression showed elevated NF-kappaB (P = 0.004) activation, a frequent molecular alteration associated with tumorigenesis and metastasis in a variety of cancers. To provide a link between p-FADD and NF-kappaB, cell culture studies demonstrated that overexpression of p-FADD leads to an increase in NF-kappaB activity and a decrease in the number of cells in the G2 phase of the cell cycle, compared with cells expressing the nonphosphorylatable form of FADD or the vector control. Furthermore, cDNA microarray analyses of lung tumor samples showed that increased levels of FADD transcripts were significantly correlated with overexpression of cyclins D1 (P < 0.01) and B1 (P < 0.01), genes that are involved in the regulation of cell cycle progression and are inducible by NF-kappaB. These studies demonstrate that induction of NF-kappaB activity and its effects on cell-cycle progression may represent a molecular basis underlying the aggressive tumor behavior associated with elevated p-FADD expression in lung adenocarcinoma.