The Oculab Tono-Pen tonometer was compared with the Goldmann applanation tonometer through studies on 197 eyes of 103 patients with glaucoma or ocular hypertension to evaluate the reliability of the Tono-Pen in determining intraocular pressure (IOP). Patients were randomized into four groups and all applanations were done by the same investigator. The Tono-Pen initially used (Tono-Pen #1) failed to calibrate after being used on 77 eyes. Tono-Pen #2 was then obtained and tested. Each Tono-Pen showed high correlation with the Goldmann readings (r greater than or equal to 0.86 in both eyes). The overall sensitivity of the Tono-Pen in detecting IOPs of 21 mmHg or higher (as measured by the Goldmann instrument) was 62.1% in right eyes, 72.4% in left eyes, with corresponding specificities of 92.6% in the right and 97.1% in the left. Given the substantial increase in sensitivity noted with Tono-Pen #2, the Tono-Pen appears adequate for screening programs where an IOP of 21 mmHg or above is considered abnormal; however, at higher IOPs (greater than or equal to 30 mmHg) the Tono-Pens tended to underestimate Goldmann IOPs. At low IOPs (less than or equal to 9 mmHg) the Tono-Pens tended to overestimate the IOPs. Perhaps with further modifications of the instrument, it could be used for clinical diagnosis and management of glaucoma patients.