The main problem observed in doctor's offices is a good air quality, especially after thermal modernization. The present research involved 15 rooms belonging to 6 different medical practices located in northeastern Poland. The observed minimum air change rate in a room equalled 0.55 ACH with the flow of ventilation air amounting to 25.20 m(3)/h, which according to standards provides a ventilation air stream sufficient for a single person. It has been proved that the temperature and humidity were conformed to the standards, whereas the CO2 concentration was close to the permissible upper limit or in some cases exceeded it. In all rooms energy consumption has been simulated for three air change rates: the real one and two recommended values: 1.5 and 3 ACH. The calculation results showed that thermal modernization could bring 18-53% energy reduction for heating without changing air quality. However in order to improve air quality it is recommended to increase air change rate in some offices so the achieved effect would be lower: only 19-28% energy consumption reduction in some tested offices or even the energy consumption would be 9-34% higher that actual in the others. The energy consumption could be reduced about 23-38% by using mechanical ventilation with recuperation. (C) 2014 Elsevier B.V. All rights reserved.