This was a 2-part study. The purpose of the 1st part was to examine the relationship between brushing force and plaque removal efficacy comparing a regular manual toothbrush (M) with an electric toothbrush the Braun/Oral-B Plak Control (B). The study consisted of a single oral prophylaxis followed by 5 experiments which differed solely in respect to toothbrushing force. At baseline (after 24-h plaque accumulation), the amount of dental plaque was evaluated and subsequently, the subject's mouth was brushed by a dental hygienist. Brushing was carried out in a random split-mouth order. Either the 1st and 3rd quadrants or the 2nd and 4th quadrants were brushed with 1 toothbrush and the 2 remaining quadrants with the other. The available time for the brushing procedure was 2 min. After brushing, the amount of remaining dental plaque was assessed. The force used in experiment 1 through 5 was 100, 150, 200, 250, 300 g, respectively. The results show that when brushing force is increased, more plaque is removed with either of the two brushes. Except for the high brushing force (300 g), the electric toothbrush removed more plaque than the manual brush. The purpose of the 2nd part was to evaluate the habitual brushing force which individuals use with various toothbrushes. Besides a manual toothbrush (M), 3 electric toothbrushes were examined, the Rotadent (R), Interplak (I) and Braun (B). 20 subjects were selected on the basis of being 'good brushers' (plaque score at screening < 25%). At baseline, each subject randomly received 1 of the 4 brushes. They were allowed a training period of 3 weeks at the end of which they were asked to abstain from brushing for at least 24 h. The plaque (Turesky modification of the Quigley & Hein) was scored, after which the subjects brushed their teeth (2 min) with the assigned toothbrush equipped with a strain gauge. A computer set-up measured (100 Hz) and calculated the mean brushing force. After brushing, the amount of remaining plaque was assessed. The design of the study was a 4-way cross-over. The results show that with a manual brush, considerably more force is used than with the electric brushes (R = 96, I = 119, B = 146, M = 273). No significant relation between brushing force and plaque removal was demonstrated for any of the brushes.