BACKGROUND:The authors conducted an in vivo study to determine if low-speed handpiece motors can become contaminated with oral flora when used with prophylaxis angles. METHODS:This crossover study involved 20 subjects, two types of handpieces and three prophylaxis angles. The authors used each handpiece/prophylaxis angle system to polish teeth. They then collected samples, spiral-plated the specimens and incubated them at 37 degrees C anaerobically and aerobically (with 5 percent carbon dioxide). After incubation, the authors examined the plates for the presence of bacterial colonies. RESULTS:At least 75 percent of the handpiece/prophylaxis angle systems used on the 20 subjects had bacterial contamination for at least one cultured area. Of the 420 specimens, 258 (61.4 percent) produced bacterial growth. Contamination varied from zero to 6,300 colony-forming units per milliliter. CONCLUSIONS:These data suggest that the internal surfaces of low-speed handpieces can become microbially contaminated during use with prophylaxis angles. CLINICAL IMPLICATIONS:Unless low-speed handpieces are sterilized properly after each use, they pose a risk for crossinfection.