Poor adherence to glaucoma medications is associated with progressive vision loss. While many interventions have sought to increase glaucoma medication adherence, the amount by which adherence must increase to have a clinically significant effect remains unknown. To generate a hypothesized minimal clinically important difference (MCID) for glaucoma medication adherence, we conducted interviews with glaucoma experts. Semi-structured interviews were conducted with members of the American Glaucoma Society. MCID was defined in two ways: 1) the incremental increase in the average percentage of eye-drops a patient takes at roughly the correct time, and 2) the incremental increase in proportion of a patient population who attain good adherence. Good adherence was defined as taking more than 80% of drops at approximately the prescribed dose time. Expert opinions on the MCID for glaucoma medication adherence and open-ended responses were recorded through field notes. 25 experts were interviewed. They estimated the MCID for average individual adherence levels as 17.7% (95% CI: 14.6, 20.8). Experts estimated the MCID for the proportion of patients in a practice who attain good adherence (defined as >80% of eye drops taken as prescribed) as 18.5% (95% CI: 15.6, 21.5). The most common identified themes were that the MCID should take into account the cost of the intervention and the burden to the ophthalmologist and to the practice, where experts thought that more costly interventions or those that required more physician time should have larger MCIDs. Based on expert opinion, we hypothesize that the MCID for glaucoma medication adherence is between 15%-20%. However, the MCID for a given intervention must take into account several factors, including intervention cost and physician burden. This hypothesis may facilitate the design and implementation of future studies to objectively determine an MCID for glaucoma medication adherence.S. Karger AG, Basel.