The p21 cyclin-dependent kinase inhibitor blocks cell cycle transition and replication in response to DNA damage. Although required for p53-mediated cell cycle arrest, p21 expression can also be initiated via p53-independent pathways. This study examines the postirradiation expression of p21 in squamous cell carcinoma (SCC) of the esophagus to determine whether this p21 production is p53-dependent or independent. We sequenced p53 exons 5-8 and the exon-intron junctions of four esophageal SCC lines, KYSEs 30, 150, 410, and 960. We exposed these same lines to increasing doses of radiation (3 to 24 Gy) and subsequently extracted their total protein. The p21 content of the protein was then determined via p21 ELISA. The same cell lines were also irradiated for determination of clonogenic survival over the course of 7 days. Cells were counted via a Coulter machine. KYSE 30 and 410 were found to have mutations in their p53 genes, while KYSEs 150 and 960 had wild-type p53 genomes. All cell lines produced basal levels of p21 (from 3.2 to 7.8 ng/ml) and all lines increased production in response to radiation (6.4 to 16.8 ng/ml at 3 Gy, P < 0.05 for all lines vs. their controls). Cells displayed dose-dependent mortality in response to radiation, with only minor differences in survival between two of the lines. All of the esophageal SCC lines studied produced basal p21 and increased p21 with irradiation. p21 production was independent of p53 status. Previous reports have failed to detect elevation of p21 expression in esophageal SCC, and this is the first report of radiation-induced p21 expression in esophageal cell lines.