We read with interest the recent study by Panchabhai et al in CHEST (May 2009),1 which examined the impact of twice-daily cleansing with 0.2% chlorhexidine on the development of pneumonia in critically ill patients. Although the overall incidence of pneumonia decreased during the study, cleansing with 0.2% chlorhexidine was not superior to the control solution, potassium permanganate. It was argued that the lack of efficacy might be explained by the use of 0.2% chlorhexidine and that higher concentrations (2%) of this drug might be effective in ICU patients as supported by data from recent studies.2,3 However, these disparate results may be explained by the mechanism of action of this antiseptic. Chlorhexidine is unusual in that it is a more effective antiplaque agent than other drugs with greater antimicrobial activity in vitro, because chlorhexidine binds to oral surfaces and is released over time. This property, known as substantivity, is well recognized among dental health care workers and depends on adherence to clean oral surfaces. Thus chlorhexidine is effective at inhibiting plaque formation in a clean mouth but is otherwise of limited value.4 Mechanical methods of cleaning, such as tooth brushing, are efficient at removing plaque, but this was not undertaken in the current study.1 In the absence of a reduction of plaque by brushing it was unlikely that the addition of 0.2% chlorhexidine would prevent nosocomial pneumonia. Plaque scores are higher among critically ill patients than healthy adults and increase over time.5 A 10-fold increase in antiseptic dose to 2% chlorhexidine may overcome the microbial burden of higher plaque scores in some patients, but in one of the supporting studies tooth brushing was also undertaken.3 The randomized controlled trials in patients undergoing cardiac surgery differ substantially from those undertaken on ICU patients because these elective patients will have brushed their teeth prior to surgery and chlorhexidine will therefore have a substantial effect.