When a flat chamber develops in an eye after filtration surgery, management depends on whether the cause is excessive filtration or aqueous misdirection (ciliary block or malignant glaucoma). This diagnosis is often based on the intraocular pressure: low pressure in excessive filtration, high pressure in aqueous misdirection. To determine the accuracy of tonometry when the lens is in contact with the cornea, flat anterior chambers were created in 5 eyes obtained from an eye bank. The pressure in the vitreous cavity was raised and lowered with an infusion line and monitored with a pressure transducer. The intraocular pressure was estimated with a Goldmann applanation tonometer, a Pneumatonometer, and a Tono-pen. The readings poorly represented the actual pressure in the vitreous cavity. The error was 0 to 51 mmHg (mean, 12.8 mmHg) with the Goldmann tonometer, 0 to 33 mmHg (mean, 9.0 mmHg) with the Pneumatonometer, and 1 to 28 mmHg (mean, 13.5 mmHg) with the Tono-pen. Therefore, in the face of a flat anterior chamber, pressure measurements made on the cornea cannot be relied on to distinguish excessive filtration from aqueous misdirection.