Objective: To study indoor environmental quality (IEQ) in elementary school buildings and its association with students' learning outcomes. Methods: Measurements of ventilation rates and temperatures were recorded during school days in 108 classrooms in 60 schools in the spring and summer of 2007; background information on 3514 school buildings was retrieved from the Finnish population register. Data on school environment and students' health were collected by questionnaires from 4248 students as well as from 1154 school principals. Results from a national student achievement assessment program were used to assess learning. Results: Upgrades to heating, ventilation, and air conditioning (HVAC) systems correlated significantly with airflow measurement, ventilation rate per student and per area, and mean temperature (rs = 0.642, rs = 0.654, rs = 0.647 and rs = -0.325 & r = 0.481, r = 0.483, r = 0.574, r = -0.271 respectively). The ventilation rate per student correlated with the number of students in classrooms (rs = -0.360 & r = -0.387) and mean temperature (rs = -0.333 & r = 0.393). Only schools with a mechanical supply and exhaust type of ventilation met the recommended ventilation rate per student of 6 l/s per person. An association was found between lower mathematics test results and schools that did not meet the recommended ventilation rate. Conclusion: Ventilation is associated with thermal comfort and students' learning outcomes. The ventilation system requires scheduled maintenance or replacement as well as ongoing ventilation adjustment to accommodate the number of students at any one time.