Computer simulations of airflow and radon transport in four large buildings were performed using CONTAM. The buildings modeled included a twelve-story multi-family residential building, a five-story mechanically-ventilated office building with an atrium, a seven-story mechanically-ventilated office building with an underground parking garage, and a one-story mechanically-ventilated school building. Inter-zone airflow rates and radon concentrations were predicted in these buildings as a function of wind speed and direction, indoor-outdoor temperature difference, and ventilation system operation. Ventilation system factors that were studied included the operation of exhaust fans in the apartment building and variations in the percent outdoor air intake in the office buildings. Simulations in the office buildings were also made with the ventilation systems off and with variations in the balance of the supply and return airflow rates.