The aim of this study was to identify predictors of intraocular pressure reduction following selective laser trabeculoplasty (SLT) in patients with primary open-angle glaucoma, who are already taking maximally tolerated IOP-lowering medication and need further IOP reduction. In this prospective case series, 157 eyes of 157 open-angle glaucoma patients who were assigned for SLT for further IOP reduction were included. Each patient had diurnal IOP measurements taken before and on average six months following SLT. The mean of six IOP measurements was compared. The following parameters were analysed for their association with SLT success: age, gender, spherical equivalent, high-pressure or normal-pressure open-angle glaucoma, number and type of pressure-lowering medications, lens status, pre-SLT IOP, IOP at the time of diagnosis, duration of glaucoma, visual ﬁeld stage and central corneal thickness. The only parameter that was predictive for absolute and relative mean diurnal IOP reduction after SLT was the preoperative mean diurnal IOP. One hundred per cent of the patients with a mean diurnal preoperative IOP of more than 18mmHg had an IOP reduction after SLT. With mean diurnal preoperative values of 14-18mmHg and with values below 14mmHg, 83.1% of the patients and 64% of the patients respectively, showed an IOP reduction. Patients with mean diurnal IOP levels below 14mmHg might not beneﬁt from the procedure at all.