Water and aerosol derived from high-powered aerosolizing instruments in dental units represents a potential source of bacterial infection especially by Legionella and Pseudomonas. In this study, water and aerosol samples were taken from 41 dental units in Istanbul. In the samples taken, the count of aerobic heterotrophic bacteria and the presence of Legionella and Pseudomonas were researched by using culture methods. The aerosol samples were collected by active sampling. Physical and chemical parameters of the samples were also measured. The number of aerobic heterotrophic bacteria in 26 dental units out of 41 (63. 41%) exceeded the acceptable limit. Gram negative rods were the predominant bacteria. Legionella spp. and Pseudomonas spp. were detected in one and eight water samples (19.5%), respectively. Pseudomonas was not detected in aerosols. The presence of Legionella in the aerosol taken from high-speed drills has been searched and detected for the first time in Turkey. These findings indicate that investigation on bacterial contamination and related risk factors in dentistry should be expanded and effective precautions should be applied in order to reduce bacterial loads.