Increasing epidemiological and experimental evidence implicates non-steroidal anti-inflammatory drugs (NSAIDs) as anti-tumorigenic agents. The precise mechanisms whereby NSAIDs exert their anti-neoplastic effects remain poorly understood. Studies from hereditary and sporadic colorectal cancer (CRC) patients suggest that NSAIDs may interfere with initiating steps of carcinogenesis, i.e. disturbances within the beta-catenin signaling pathway. We therefore investigated beta-catenin/TCF signaling in response to aspirin or indomethacin, respectively, in four CRC cell lines (SW948, SW480, HCT116, LoVo). Both, aspirin and indomethacin inhibited transcription of a beta-catenin/TCF-responsive reporter gene in a dose dependent manner. In addition, the beta-catenin/TCF transcriptional target cyclin D1 was downregulated by both drugs. Endogenous beta-catenin levels remained unaffected by either drug. Moreover, indirect immunofluorescence studies revealed no significant changes of subcellular beta-catenin localization in either cell line after NSAID treatment. Likewise, binding of the beta-catenin/TCF complex to its specific DNA-binding sites was not altered, as demonstrated by electrophoretic mobility shift assay (EMSA) of nuclear extracts derived from NSAID treated cells. These results strongly suggest that aspirin and indomethacin attenuate the transcription of beta-catenin/TCF-responsive genes, by modulating TCF activity without disrupting beta-catenin/TCF complex formation.