Ocular surface disease (OSD) is common in patients chronically treated for glaucoma. This may be related to the drug itself but often to the preservatives in the medication. Much work has been done on the most common preservative, benzalkonium chloride (BAK). The preservatives cause microbial cell destruction by disrupting the cell membrane lipids and cytoplasmic contents. The lipid-destruction properties of BAK also affect the tear lipids responsible for the stabilisation and evaporation control of the tear film; its disturbance being a major cause of dry eye. This paper describes a prospective, case-controlled study of 130 participants, including normal controls, glaucoma patients on chronic preserved anti-glaucoma medication and post-trabeculectomy patients not on medication for over six months. Compared with normal controls, both groups of glaucoma patients were more likely to have a raised tear film osmolarity (TFO), with an odds ratio of 4.43 and 2.76 respectively. Both groups of glaucoma patients were also more likely to have dry eye symptoms. The authors conclude that TFO is increased in glaucoma patients on chronic topical medications and remains elevated after trabeculectomy surgery. They suggest further studies may be warranted to determine whether dry eye is merely aggravated by or can be caused by trabeculectomy surgery. It will be interesting to see what effect the increasing use of non-preserved eye drops has on TFO and symptoms of dry eye.