Objectives: Dental units are necessary tools for modern dentistry. Microbial contamination of dental units is thought to be the result of biofilm formation in various parts of the unit. We aimed to identify the total microbial count in the suction hose of dental units, and detect the presence of Pseudomonas aeruginosa. Methods: Random sampling of suction hoses of dental units in Birjand, Iran, was performed in dental clinics on the first (n = 115) and last (n = 115) working days of the week. Total viable counts of bacteria and detection of P. aeruginosa were performed on plate count agar and cetrimide agar, respectively. Plates were incubated at 37 °C for 48 hours. Results: None of the samples were free from bacterial contamination. P. aeruginosa was detected in 22.6% and 18.3% of samples taken on the first and last working days, respectively. Conclusions: Suction hoses were heavily contaminated with bacteria, especially opportunistic pathogens, and current disinfection does not adequately control the contamination.