Experimental and clinical evaluation was made of a pneumatic tonometer developed for measuring intraocular pressure. In manometric studies on dead human eyes, a direct proportionality was found between intraocular pressure and average readings by pneumatic tonometry on either the cornea or sclera. From pressure measurements made with the pneumatic tonometer during open and closed stopcock manometry, the total volume of corneal and scleral indentation was calculated to be 1 to 2 μl. A positive correlation was found between measurements of pressure made by Goldmann, Schiøtz, and pneumatic tonometers, but significant differences were found in individual subjects. When seated patients assumed a supine position, the pneumatic tonometer showed an average intraocular pressure increment of 1.7 mm Hg. The cause of these differences and the problems of making accurate pressure measurements by the three tonometric procedures are discussed.