Many reported series of patients treated with topical Timolol for chronic open angle glaucoma have been changed from other medication after a 'wash-out' period. This four year study reports the ocular hypotensive effect of Timolol in 82 new patients with chronic open angle glaucoma followed for up to 4.5 years. The effects on intraocular pressure, visual acuity, visual field, pupil size, pulse rate, blood pressure and Schirmer's test were recorded and correlated. Corneal sensitivity was also followed in some patients, matched by a series of controls, using the method of Cochet and Bonnet. The results are discussed, and only slight reduction in blood pressure and pulse rate was found, and there was no significant effect on the pupil diameter or Schirmer's test. There was no significant effect on corneal sensitivity. There was only slight tachyphylaxis. Of the 82 patients, 20 patients required extra medical or surgical treatment. 16 patients dropped out through death or failure to attend, and of the remaining 66 patients, 69.7 per cent were controlled by Timolol alone.