The Laser in Glaucoma and Ocular Hypertension (LiGHT) Trial demonstrated the efficacy and safety of selective laser trabeculoplasty (SLT) compared to topical hypotensive medication as 1st-line therapy for ocular hypertension and open angle glaucoma. This sub-study explores the impact of pre-treatment (baseline) intraocular pressure (IOP) on treatment response for SLT and medication. Post hoc analysis of randomised control trial data. 1146 eyes from 662 patients were included in this analysis: 559 eyes in the SLT group and 587 in the medication group. IOP reduction at 8 weeks following treatment with either SLT or prostaglandin analogue (PGA) eye drop initiation was assessed at different levels of baseline IOP, and the groups were compared. Differences in absolute and percentage IOP lowering between SLT and PGA medication were tested with a linear mixed effects model. Differences in the probability of achieving ≥20% IOP lowering between SLT and PGA medication, at different levels of baseline IOP, was estimated using a logistic mixed effects model. IOP lowering response to SLT versus PGA eye drops. Mean IOP was not significantly different between the groups, at baseline or 8 weeks following treatment initiation. Both treatments showed greater IOP lowering at higher baseline IOP and less IOP lowering at lower baseline IOP. SLT tended to achieve more IOP lowering than PGA drops at higher baseline IOP. PGA drops performed better at lower baseline IOP, and the difference compared to SLT, in terms of percentage IOP reduction, was significant at baseline IOP ≤ 17 mmHg. There was a significant difference in the relationship between baseline IOP and probability of ≥20% IOP lowering between the two treatments (p = 0.01), with SLT being more successful than PGA at baseline IOP > 22.51 mmHg. These data confirm previous reports of greater IOP lowering with higher baseline IOP for both SLT and topical hypotensive medication. In treatment naïve eyes, at higher baseline IOP, SLT was more successful at achieving ≥20% IOP lowering than PGA drops. At lower baseline IOP, a statistically greater percentage, but not absolute, IOP lowering was seen with PGA drops compared to SLT, although the clinical significance of this is uncertain.