To investigate whether aspirin is able to augment gemcitabine-induced cytotoxicity in human pancreatic cancer cells.Two gemcitabine-insensitive human pancreatic cancer cell lines, PANC-1 and Capan-1, were used. Cells were treated with either aspirin or gemcitabine alone or both of them. Cell growth and apoptosis were determined by MTT assay, Annexin V or Hoechest 33258 staining. Cell cycle distribution was examined by flow cytometry. Western blot with specific phosphorylated protein antibodies was used to detect the activation of protein kinase. RT-PCR and Western blot were applied to assess the transcription and protein level for cyclin D1 and Bcl-2.Aspirin alone significantly inhibits the proliferation of PANC-1 cells by causing cell cycle arrest at G(1) phase. Aspirin potentiates the anti-survival effect of gemcitabine as well as its pro-apoptotic effect in PANC-1 cells, although aspirin per se does not trigger apoptosis. Aspirin inhibits GSK-3beta activation and suppresses the expression of its downstream gene products (cyclin D1 and Bcl-2), which are implicated in proliferation, survival and chemoresistance of pancreatic cancer. The effects of aspirin on Capan-1, were similar to that on PANC-1.Our results suggest that aspirin inhibits the proliferation of gemcitabine-resistant pancreatic cancer cells and augments the antisurvival effect of gemcitabine, probably by suppressing the activity of GSK-3beta and its downstream gene products.