The significance of tear film break-up time (BUT) was investigated by analyzing its distribution and reproducibility in 30 normal subjects and 20 dry eye patients. The BUTs* were all above 5 seconds and the reproducibility was 65% in normalsubjects. However, those were all below 10 seconds and the reproducibility was 95% in dry eye patients. These results suggest that: 1) The patient whose BUT* is below 5 seconds can be diagnosed as dry eye syndrome. 2) The value of BUT* above 10 seconds can be regarded as normal. (*The mean value of 4 different visits)