PURPOSE: Infestation with demodex mites has been linked to the development of chalazion, meibomian gland dysfunction, and blepharitis. An effective treatment is the eyelid application of terpinen-4-ol (T4O), a tea tree oil component. However, T4O is also known to be toxic to nonocular epithelial cells. We hypothesize that T4O toxicity also extends to human meibomian gland epithelial cells (HMGECs). METHODS: Immortalized (I) HMGECs were cultured with varying concentrations (1.0%-0.001%) of T4O under proliferating or differentiating conditions up to 5 days. Experimental procedures included analyses of cell appearance, survival, P-Akt signaling, lysosome accumulation, and neutral lipid content. RESULTS: Our findings show that T4O causes a dose- and time-dependent decrease in the cell survival of IHMGECs. After 15 minutes of exposure to 1% T4O, IHMGECs exhibited rounding, atrophy, and poor adherence. Within 90 minutes of such treatment, almost all cells died. Reducing the T4O concentration to 0.1% also led to a marked decrease in P-Akt signaling and cell survival of IHMGECs. Decreasing the T4O amount to 0.01% caused a slight, but significant, reduction in the IHMGEC number after 5 days of culture and did not influence the ability of these cells to differentiate. CONCLUSIONS: T4O, even at levels 10-fold to 100-fold lower than demodicidal concentrations, is toxic to HMGECs in vitro.