This study covering a period of almost 16 weeks shows that manual tooth cleaning must be regarded as somewhat superior to electric tooth-brushes in terms of the plaque index. Concerning the more important sulcus bleeding index, the electric tooth-brush leads to better results. This, however, only after at least two weeks of "familiarisation" of the patients with the electric tooth-brush, and after a minimum follow-up time of three to five weeks! This is why another quite important conclusion should be drawn from this study: studies of this type should not be planned for too short a period, i.e. for at least 12 weeks.