The aim of this clinical pilot study was to compare the effect of tea tree oil with the effect of water and chlorhexidine on supragingival plaque formation and vitality. Eight subjects were asked to refrain from any kind of mechanical oral hygiene for 4 days after professional tooth cleaning (day 0), and to rinse with water instead for 1 week, with chlorhexidine in a second and tea tree oil in a third test week. The plaque index (PI), which was evaluated daily (days 1-4), served as a clinical control parameter. On the last day of the study (day 4), the plaque covering the front teeth was stained, photographed, and therefrom the plaque area (PA; %) was estimated using a digital measuring system. Each day of the study (days 1-4), the sampled plaque was examined using a vital fluorescence technique. Tea tree oil reduced neither the clinical parameters (PI and PA) nor the vitality of the plaque flora significantly. Within the limitations of the study design, it was determined that a solution with tea tree oil--utilized as ordinary mouthwash--has no positive effect on the quantity or quality of supragingival plaque.