OBJECTIVE To detect the mechanism of intraocular pressure elevation during hemodialysis. METHODS We measured intraocular pressure, as well as serum osmolality and plasma CO2 pressure, every 30 min during hemodialysis, in 5 eyes with severely compromised aqueous outflow facility (Group A) from 4 renal failure patients. The same measurements were repeated on the same eyes using intravenous hyperosmotic Glyceol to prevent a rapid change in serum osmolality. We also measured the same parameters on 8 eyes with normal aqueous outflow facility (Group B) from 5 patients. The mean +/- SE of percent changes in each parameter was used for the statistical analysis. RESULTS In Group A, the mean percent change of intraocular pressure increased significantly after 90 min, with the exception of the change at 180 min. The mean percent change of serum osmolality decreased significantly after starting dialysis. A negative correlation in the mean percent change of intraocular pressure with serum osmolality was detected (r = -0.759, r < 0.0001). The administration of intravenous hyperosmotic agent prevented significant changes in not only serum osmolality but also intraocular pressure. In Group B, the mean percent change in intraocular pressure showed no significant difference at any time, although the change in serum osmolality decreased significantly. CONCLUSIONS A remarkable rise in intraocular pressure occurs during hemodialysis in eyes with an impaired aqueous outflow, when serum osmolality decreases rapidly.