The purpose of this study is to determine whether 24-hour fluctuation of intraocular pressure (IOP) is associated with central corneal thickness (CCT) in subjects with ocular hypertension or open angle glaucoma and in age-matched controls. Also to evaluate whether mean IOP reduction as a response to latanoprost (0.005% Xalatan) is associated with CCT, after a 4-weeks period of treatment.