The effects of an oligomer, urethane dimethacrylate (UDMA), on two human cell lines were studied using flow cytometry (FCM). Untreated and treated cultures of propidium iodine-stained KB (epidermal oral carcinoma cells) and human foreskin fibroblast (HFF) cells were analyzed for cellular DNA content. Concentrations of 10 and 25 pM of UDMA slightly perturbed the KB cell cycle progression at 24 and 48 h of incubation. However, the effect of 50 pM was more pronounced at the latter incubation time period. In cell growth experiments, the sublethal concentrations (10 and 25 pM) produced inhibition of KB cell growth rate at a moderate level, which resulted in the prolongation of cell population doubling time. Significant inhibition of cell growth occurred when 50 pM (lethal concentration) was used. Data obtained from the cell cycle perturbation analysis, evidenced by FCM, correlated with the extent of inhibition in KB cell growth rates. The effects of sublethal concentrations were reversible during a 24 h period of oligomer withdrawal from culture medium. In contrast, the effects of 50 pM were not reversible. In HFF cells the depletion of S phase in the cell cycle was the major effect of 50 pM of UDMA. It was concluded that FCM technology is an ideal and practical approach for studying the cytotoxicity of components of dental composites.