Surface tension has been measured by the method of Ferguson and Kennedy on small (0.3-0.4 microliter) samples of tears from 65 normal and 35 dry eyes. Non-invasive break-up time (NIBUT) was also measured on the same patients as an indication of tear film stability. The mean (+/- SD) surface tension value was 43.6 +/- 2.7 mN/m for normals, and 49.6 +/- 2.2 mN/m for dry eyes. All NIBUT values for dry eyes were below 20 sec (8.9 +/- 5.1 sec, mean +/- SD, n = 35) while 53% of normal values were 30 sec or over. A negative correlation was found between surface tension and NIBUT for both dry eyes and normals. Comparison of surface tension results for tears and a variety of standard solutions indicated that mucus makes the greatest contribution, even at low concentrations, but the effect of proteins can be large, especially in inflammatory states where serum proteins leak from conjunctival vessels. The measurement of surface tension is simple and direct, and in conjunction with the NIBUT value can help in classification of tear quality in questionably dry eye cases.