The prostaglandin derivatives are thought to lower intraocular pressure (IOP) primarily by increasing uveoscleral outflow. The ocular side effect, hyperemia, appears to occur via a secondary, unrelated mechanism. Variations in the IOP-lowering effect and incidence of hyperemia associated with these drugs are a function of their different chemical structures. Among the currently approved prostaglandin derivatives, hyperemia occurs in as many as 50% of patients treated with travoprost and as few as 5% of patients treated with latanoprost. The side effect of hyperemia may be of concern to the ophthalmologist for at least 2 reasons: hyperemia may compromise the outcome of filtration surgery, and it may represent a cosmetic problem to the patient thereby leading to non-compliance. The extent to which hyperemia may contribute to patient noncompliance and the effect of administration of the prostaglandin derivatives on outcome of filtration surgery remain to be determined. Until more definitive data are available, when selecting a prostaglandin analogue for ocular hypotensive therapy, it seems prudent to choose an agent with a low incidence of hyperemia.