The intraocular pressure reducing effect and side-effects of latanoprost, a phenyl-substituted prostaglandin analogue, were compared with those of timolol, in a group of 31 glaucomatous or ocular hypertensive patients, divided into three subgroups. The study was randomized and double masked. At the end of 6 month's treatment with latanoprost 0.005% once daily, either as a morning dose or as an evening dose, there was a reduction in intraocular pressure of 33% (p < 0.001) and 36% (p < 0.001), respectively. The intraocular pressure reduction of timolol 0.5%, administered twice daily was 26% (p < 0.001). There was no significant difference in conjunctival hyperemia between the groups and there were few subjective symptoms in any of the patients. One patient with a light green-brown iris, treated with latanoprost in one eye only, exhibited an increase in iris colour in the treated eye at week 26, and did not show any signs of reversion 9 months after discontinuing the therapy. It may be concluded that latanoprost is well tolerated and at least as effective as timolol in reducing intraocular pressure in patients with glaucoma or ocular hypertension when applied once daily. The exact mechanism behind the increase in iris pigmentation and the clinical significance of this previously unknown side-effect needs to be investigated further.