Cyclin D1 plays an essential regulatory role in the G1 phase of the cell cycle. The cyclin D1 gene is amplified in 20-50% of squamous cell carcinomas (SCCs), and the protein is overexpressed in up to 80% of SCCs. Our hypothesis was that gene transduction of antisense (AS) cyclin D1 in human SCCs in vivo would result in tumor reduction. A cyclin D1 cDNA was inserted into an E1/E3-deficient serotype 5 adenovirus (AS cyclin D1) in an AS orientation using homologous recombination. AS cyclin D1 transduction suppressed cyclin D1 protein expression in both cultured cells and tumors. AS cyclin D1 significantly inhibited cell proliferation by both [3H]thymidine incorporation in six SCC cell lines (P = 0.01-0.001) and the conversion of tetrazolium salt to formazan in four SCC cell lines (P = 0.01-0.004). Apoptosis detected in >25% of cells in each cell line 48 h after AS cyclin D1 transduction paralleled the reduction in cyclin D1 protein. Preformed SCCs transduced with AS cyclin D1 were significantly inhibited (P = 0.002-0.005), and apoptosis was prominent in the AS cyclin D1-treated tumors, but not in tumors treated with the control vector. These data extend prior in vitro and ex vivo results and indicate that AS cyclin D1 suppresses SCC growth both in vitro and in vivo through suppression of cyclin D1 protein expression, leading to cellular apoptosis. Our findings suggest that cyclin D1 may have a role in cell survival and that cyclin D1 AS therapy may be useful as an adjunct to standard treatment for SCC.