We have previously reported that elevated fibroblast growth factor-2 (FGF-2) expression is associated with tumor recurrence and reduced survival after surgical resection of esophageal cancer, and that these risks are reduced in tumors co-expressing an endogenous antisense (FGF-AS) RNA. In the present study we examined the role of the endogenous FGF-AS transcript in the regulation of FGF-2 expression in the human lung adenocarcinoma cell line, Seg-1. FGF-2 and FGF-AS were temporally and spatially co-localized in the cytoplasm of individual cells, and knock-down of either FGF-2 or FGF-AS by target specific siRNAs resulted in dose-dependent up-regulation of the complementary transcript and its encoded protein product. Using a luciferase reporter system we show that these effects are mediated by interaction of the endogenous antisense RNA with the 3UTR of the FGF-2 mRNA. Deletion mapping identified a 392 nt sequence in the 5823 nucleotide FGF-2 untranslated tail which is targeted by FGF-AS. siRNA-mediated knockdown of either FGF-AS or FGF-2 significantly increased the stability of the complementary partner mRNA, demonstrating that these mRNAs are mutually regulatory. Knockdown of FGF-AS also resulted in reduced expression of argonaute-2 (AGO-2) and a number of other elements of the endogenous microRNA/RNAi pathways. Conversely, siRNA-mediated knockdown of AGO-2 significantly increased the stability of the FGF-2 mRNA transcript, and the steady-state levels of both FGF-2 mRNA and protein, suggesting a role for AGO-2 in the regulation of FGF-2 expression.