Thirty-seven patients with ocular hypertension (OH) and 29 age matched controls were studied. Intraocular pressure (IOP) measurements were made in the sitting position and, immediately after changing posture, in the lying position. Sixty-three (85%) of the OH eyes and all the control eyes showed either stable IOP readings on changing posture of small elevations up to 3 mmHg. Eleven (15%) of the OH eyes showed larger elevations of IOP, between 5 and 9 mmHg when the patients changed to the lying position. We suggest that an IOP measurement in the lying position should be included in the routine evaluation of the patient with ocular hypertension.