Abstract: Introduction Critically ill patients cannot clean their own teeth. A variety of methods are used but as the best method is unknown we performed a prospective randomised double-blind controlled trial of three methods.Method Of 359 consecutive admissions to the Department of Critical Care Medicine between 31/01/97 and 25/05/97, 222 were excluded (62 edentulous, 6 unexaminable, 142 transferred alive and 12 dead or dying at 24 hours). The remaining 137 patients had quantitative (picture-linked, ordinal score) assessment of caries, peridontal status and plaque (in 12 segments of teeth) before randomisation (to the use of either toothbrush, jumbo swab or sonic toothbrush) by pre-assigned sealed envelopes. All teeth were cleaned (prescribed four hourly) with 0.2% chlorhexidine solution. Daily plaque scores were obtained (by an assessor (SC) blind to treatment allocation) until withdrawal, death or transfer.Results Toothbrush Jumboswab SonicPatients assigned 50 48 39Withdrawn within 24hrs. 9 3 5Patients remaining 41 45 34Percentage of teeth segments thatare pristine: pre treatment 45 50 51 after 2 days 74 57 90 F(2.63) = 5.00 p = 0.0097 More withdrawals for patient noncompliance after randomisation occurred in the sonic group (5/34 versus 3/86). Conclusion. By the second day the sonic toothbrush was the most effective in plaque removal with the toothbrush the next most effective method