Aims: To assess the additive effect of unoprostone and latanoprost in patients with primary open angle glaucoma (POAG) or ocular hypertension (OHT) Methods: 32 patients with POAG or OHT were randomised to receive either latanoprost once daily or unoprostone twice daily for 4 weeks. After 4 weeks, all patients received both latanoprost and unoprostone for another 4 weeks. The IOP was measured at 9 am and 5 pm on the baseline, day 28, and day 56 visits, and at 9 am on day 14 and day 42 visits. The medications were given to the patients in an open label fashion. The observer was masked to the treatment given. The mean of the measurements was calculated. Safety parameters were also recorded. The additive effect of the medications was assessed by the reduction in intraocular pressure (IOP) when both medications were used, compared with when one medication was used. Results: 28 patients completed both treatment periods and had IOP data available for evaluation. After 1 month of treatment, latanoprost significantly reduced IOP (mean by 6.1 (SEM 0.8) mm Hg (p<0.001) and unoprostone by 4.9 (1.0) mm Hg (p<0.001) from the baseline of 24.4 (0.6) mm Hg and 24.4 (1.1) mm Hg respectively (p = 0.18). When latanoprost once daily was given to patients treated with unoprostone, there was additional IOP lowering of 1.9 (0.6) mm Hg (p = 0.012). However, adding unoprostone to those being treated with latanoprost produced an IOP change of +0.4 (0.5) mm Hg (p = 0.42). Ocular symptoms and findings were mild and equally distributed between treatment groups, and after combined therapy. Hyperaemia and ocular irritation were the most frequently reported events. Over a third of patients experienced ocular irritation with the combination of medications. Conclusions: Latanoprost once daily causes additional IOP lowering in eyes which were being treated with unoprostone twice a day. However, there was no additional IOP lowering when unoprostone was added to eyes which were being treated with latanoprost. Both drugs were well tolerated together with few ocular adverse events.