The indoor air climate of an underground train station was investigated during two days in January 2008. The underground platform was accessed from ticket halls on each side with air volumes of 1 000 m3 and 1 430 m3, respectively. The station platform air volume was approximately 14 300 m3. Air from the outside could enter either via the ticket halls or via the train tunnels from ventilation towers situated on each side of the platform area. The local mean age of air was determined in several locations at different heights using pumped sampling and homogeneous emission of PFTs. In addition, the temperature and relative humidity was measured at selected locations. The average air exchange rate per hour (ACH) was found to be 3.62 h-1, ranging from 4 h-1 at rush hours to slightly more than 3 h-1 at night and in the middle of the day. The largest ACH (4.5 h-1) was found at rush hour in one of the ticket halls, corresponding to a flow rate of 75 000 m3/h. The lowest ACH (2.8 h-1) was found in the other ticket hall at night, corresponding to a flow rate of 47 000 m3/h. In the middle of the station platform the ACH was lower than the ACH at the platform ends.