This study estimated the health, energy and economic benefits of an economizer ventilationcontrol system that increases outside air supply during mild weather to save energy. A modelof the influence of ventilation rate on airborne transmission of respiratory illnesses was usedto extend the limited data relating ventilation rate with illness and sick leave. An energysimulation model calculated ventilation rates and energy use versus time for an office buildingin Washington, D.C., with fixed minimum outdoor air supply rates, with and without aneconomizer. Sick-leave rates were estimated with the disease transmission model. In themodelled 72-person office building, our analyses indicate that the economizer reduces energycosts by approximately $2000 and, in addition, reduces sick leave. The financial benefit of thedecrease in sick leave is estimated to be between $6000 and $16 000. This modelling suggeststhat economizers are much more cost effective than currently recognized.