The lip vermillion constitutes a transition tissue, between oral mucosa and skin, where oral mucosal cells from epithelial and connective tissue compartments are exposed to ultraviolet (UV) sunlight. Fibroblasts are abundant resident cells of the connective tissue which are key regulators of extracellular matrix composition, as well as, epithelial and endothelial cell function. UVB light, an inherent component of sunlight, causes several alterations in skin fibroblasts, including premature senescence and increased cyclooxygenase (COX)-2 expression. To assess if UVB irradiation had similar effects on fibroblasts derived from human oral mucosa (HOM), primary cultures of HOM fibroblasts were irradiated with a single dose of 30 or 60 mJ/cm 2 of UVB light or shamirradiated. Fibroblast proliferation was assessed from 3 to 48 hrs after UVB-irradiation utilizing [ 3 H]-thymidine incorporation and MTT assays. In addition, COX-2 mRNA expression was detected by RT-PCR, and PGE 2 production was assessed using enzyme immunoassay from 0.5 to 24 hrs after UVBirradiation. The results showed a significant decrease in proliferation of UVB-irradiated HOM fibroblasts as compared to controls as measured by both [ 3 H]-thymidine incorporation and MTT assays (p<0.001). HOM fibroblasts had increased COX-2 mRNA expression at 0.5 and 12 hrs after irradiation, and PGE 2 production was elevated at 12 and 24 hrs post-irradiation as compared to controls (p<0.05). The results showed an inhibitory effect of a single dose of UVB irradiation on HOM fibroblast proliferation with an increase in COX-2 expression and activation. Therefore, photodamaged fibroblasts may play and important role in the pathogenesis of UV-induced lesions of the lip.