Programmed cell death 4 (Pdcd4), originally identified as an inhibitor of murine cellular transformation, inhibits protein synthesis by directly interacting with eukaryotic initiation factor 4A (eIF4A) of the translation initiation complex. The relevance of Pdcd4 to a broad range of human cancers derived from multiple tissue sites is unknown. Protein expression patterns from the National Cancer Institute drug-screening panel of 60 human cancer cells (NCI60) were analyzed by Western blot methods and revealed frequent reduction of Pdcd4 protein levels in renal-, lung-, and glia-derived tumors. Greater than mean Pdcd4 protein levels correlated with the antitumor activity of geldanamycin and tamoxifen. Stable expression of antisense PDCD4 significantly reduced the sensitivity of MCF-7 breast cancer cells to geldanamycin and to tamoxifen. Sensitivity to geldanamycin significantly increased in UO-31 renal cancer cells expressing sense PDCD4 cDNA. Increased geldanamycin sensitivity was accompanied by enhanced cell cycle arrest and apoptosis. One primary mode of inactivation of Pdcd4 in human cancers appears to involve down-regulated expression, and this down-regulation causes a decreased sensitivity to geldanamycin cytotoxicity. Thus, up-regulating Pdcd4 expression may be promising for geldanamycin-based combination therapy.