The rate of water evaporation from the tear film depends on the conditions under which it is determined. If measured in a controlled and easily reproducible setting, tear evaporation rate could serve as a useful diagnostic and research tool. We have developed a tear evaporimeter that provides a non-invasive method for evaluating water evaporation rate in eyes with normal or pathologic tear films. The subject is fitted with a pair of swimmer's goggles modified so that the air over the test eye is conditioned to a desired baseline of relative humidity (29.5%) and temperature (23 +/- 1 degree C). To eliminate other sources of water, the lids and skin under the goggles are covered with petroleum jelly. The subject is asked to keep his/her eye open and to avoid blinking during the 1-min test. The increase in relative humidity due to water evaporation from the tears is then determined. The surface area of the eye exposed to evaporation is calculated from the lid aperture. We measured the water evaporation rate in 52 normal eyes (average = 4.07 X 10(-7) +/- 0.40 X 10(-7) g/cm2/sec) and compared the results with values obtained in 52 eyes with various tear-film abnormalities (average = 8.17 X 10(-7) +/- 2.65 X 10(-7) g/cm2/sec). The difference between the two groups is statistically significant (P less than 0.01). In normal eyes, after the instillation of one drop of 0.5% proparacaine, the increase in evaporation rate was statistically significant.