Tear film breakup time (BUT) was measured in 25 normal subjects on 8 different occasions over a period of one month. Reproducibility of the BUT test and factors involved in interpretation of tear film breakup were studied. It was found that of 50 eyes, the average BUT ranged from five to 100 seconds, and that there were noticeable variations (P less than or equal to .05) in an individual eye from one patient visit to the next. The BUT is not a closely reproducible phenomenon in an individual eye.