The aim of the present study was to investigate the relationships between hyperosmolarity and BAK in vitro through their respective and combined proapoptotic and pro-oxidative effects on conjunctival epithelial cells, to determine whether BAK toxicity would be enhanced in hyperosmotic conditions, mimicking a frequent clinical condition in which dry eye patients receive artificial tears preserved with a chemical compound that may both promote and be potentiated by tear hyperosmolarity. This evidence concerns the gene BAK1 and Keratoconjunctivitis sicca.