The air-exchange rate is often low in schools. The authors studied the possible impact of improving school ventilation on health and exposure of pupils. Questionnaire data on allergies, asthma, and asthmatic symptoms were obtained in 1993 and 1995 for 1,476 primary- and secondary-school pupils in 39 randomly selected schools. Various exposure factors were measured in 1993 and 1995 in approximately 100 classrooms. In 12% of the classrooms, new ventilation systems were installed between 1993 and 1995; the subsequent air-exchange rate increased and the relative humidity and concentration of several airborne pollutants were reduced compared with classrooms in nonimproved buildings. The reporting of at least one asthmatic symptom and the reporting of more asthmatic symptoms in 1995 than in 1993 were less common among the 143 pupils who attended schools with new ventilation systems.