Video recordings were obtained of 90 adults aged 18-22 yr brushing their teeth. Thirty subjects were unaware they were being filmed and 60 subjects had been informed that they would be filmed while they brushed their teeth. From repeated viewing of the tapes the areas of the mouth that were brushed, the total brushing time and the proportion of time spent brushing each of 16 areas of the mouth were obtained for each subject. Subjects who knew they were being filmed brushed significantly more mandibular occlusal surfaces, and lingual areas in both arches than subjects who were unaware they were being filmed. There was no significant difference in the mean toothbrushing time between the two groups. The informed group spent proportionally less time brushing posterior buccal areas and more time on occlusal and lingual areas than the group who were unaware they were being filmed, the difference being statistically significant for the mandibular arch areas. It was concluded that knowledge of filming alters toothbrushing behaviour to a small extent so that care should be taken when interpreting behavioural changes in future intervention studies.