We developed a simple device to measure the water evaporation from the ocular surface, and we measured the evaporation rate in 18 normal individuals. We compared this to a group of 15 patients with dry eye from low tear production. We found the normal rate of water evaporation from the ocular surface to be 14.7 +/- 6.4 x 10(-7) g/cm2/s or 0.14 +/- 0.06 microliter/min at 30% relative humidity. For dry eye patients, the rate was significantly increased to 47.6 +/- 20.1 x 10(-7) g/cm2/s or 0.43 +/- 0.19 microliter/min (p < 0.005). Neither group showed any evidence of meibomian gland dysfunction that might have increased evaporation. From these results, we conclude that evaporation is accelerated when tear production decreases and is of sufficient magnitude to exacerbate the dry eye.