Esophageal adenocarcinoma (SKGT-2, SKGT-4, and SKGT-5) and epidermoid carcinoma (HCE-4) cells containing variable retinoblastoma (Rb), cyclin D1, p16, and p53 expression patterns were exposed to the synthetic flavone, flavopiridol. The IC50 was approximately 100-150 nM for each of these cell lines. Exposure of esophageal carcinoma cells to 300 nM flavopiridol induced cell cycle arrest and apoptosis, resulting in a 90% inhibition of proliferation relative to that of nontreated cells after a 5-day exposure to the drug. Western blot analysis revealed diminution of cyclin D1, Rb, and p107 protein levels after flavopiridol exposure. Whereas cell cycle arrest and overall growth inhibition did not correlate in any obvious manner with the genotype of these cell lines, apoptosis seemed to be more pronounced in SKGT-2 and SKGT-4 cells that lack Rb expression. Pretreatment of esophageal cancer cells with 9-cis-retinoic acid did not substantially potentiate flavopiridol activity in these cell lines. Although the precise mechanism of flavopiridol-mediated cytotoxicity has not been fully defined, this drug is an attractive agent for molecular intervention in esophageal cancers and their precursor lesions; further evaluation of flavopiridol in this clinical context is warranted.