To verify environmental contamination by collecting microorganisms from dental equipment surfaces before, during and after clinical procedures. Sterile swabs were used to collect the samples from dental-chair push buttons, 3-in-1 syringes, X-ray tubes, computer keys, doorknobs, and light handles before (P1), during (P2) and after (P3) clinical procedures, in the undergraduate clinic of the Dental School of Piracicaba, University of Campinas, Brazil. These samples were spread on BHI agar and incubated at 37 degrees C for 24 hours. Other dishes containing the same material were incubated at 37 degrees C in 10% CO2 for 48 hours. The resulting microorganisms were counted and classified using the Gram staining and biochemical tests. Microorganism counts among the periods and groups were analyzed by Kruskal-Wallis test (alpha=5%). The most prevalent microorganisms were Viridans group streptococci, Sthaphylococcus epidermidis and Baccillus subtilis. Push buttons were the most contaminated (P< 0.05). Microorganisms were most prevalent during clinical procedures (P< 0.05). Microorganism counts were higher in P2 than in P1 and P3 (P< 0.05). Clinical activity caused an increase in the number of environmental microorganisms, where Viridans group streptococci were the most prevalent contaminant found on equipment surfaces.18340-4