The plaque situation on 48 surfaces in 31 children 13 years of age was assessed. A re-examination within 2 hours indicated a significant increase in the number of O Plaque Index scores. Following paired comparison of each of the 48 areas, 22 % of the surfaces were found to have changed score. Significantly more scores indicating improvement of the plaque situation were found at the second examination. A re-examination of plaque on consecutive days in a similar group of children showed no significant changes. Re-examination of the gingival conditions within 2 hours indicated a significant increase in the number of scores of 2 and this was confirmed by the paired comparison on the 48 surfaces. The findings indicate systematic errors rather than intra-examiner inconsistency.