Uranium mills, as a part of the nuclear fuel cycle, produce large volumes of wastes which contain both the long and short-lived radionuclides from the naturally-occurring uranium decay chain. A relatively short-lived member of the chain, radon-222, is a noble gas and can diffuse from the wastes and be transported from its point of origin by prevailing winds. The airborne radon-222 decays into other short-lived radioactive progeny which may result in human inhalation exposures at considerable distances from the point of origin. Since the parent of radon-222 has a half-life of 1620 years, exposure to radon and its progeny will persist for thousands of years. This study was made to estimate the annual average radon-222 concentrations around the former Shiprock, New Mexico uranium mill site in order to identify areas which may require limited or restricted occupancy and to evaluate the effectiveness of the site decontamination activities in reducing the ambient airborne radon-222 concentrations. Using on- and off-site meteorological data and radon source term estimations, average annual radon-222 isoconcentration lines were determined by computer modeling for the mill site area. The lines were determined for both pre- and post-decontamination periods.