The paper discusses a case study of radon diagnostics and mitigation performed by EPA in a New York State school building. Research focused on active subslab depressurization (ASD) in the basement and, to a lesser degree, the potential for radon reduction in the basement and slab-on-grade (SOG) sections using the heating, ventilating, and air-conditioning (HVAC) system. Based on radon diagnostic measurements in the basement, a five-point ASD system was installed, and recommendations were made to increase the outdoor air supply through the basement unit ventilator. Because of the high radon levels in the basement (1720 Bq/cu m) and limited subslab pressure field extension, both mitigation approaches were needed to reduce radon to below the current EPA guideline of 148 Bq/cu m. The effects of excavating a suction pit under each of the five suction points were also investigated. Pit excavation, together with adjustment of the airflows at the suction points, decreased average radon levels in the basement by an additional 40%. In the SOG section, it was recommended that the school hire an HVAC contractor to evaluate the unit ventilators for increased outdoor air supply.