Destruction of the core of Three Mile Island Unit 2 (TMI-2) resulted in the release of large quantities of primary coolant into the reactor, auxiliary, and fuel handling buildings. Volatile fission products, mainly noble gases, with lesser amounts of radioiodines, escaped into the plant atmosphere and were discharged to the environment via the stack from the plant ventilation system. The contaminated water was retained within the buildings, from which no uncontrolled releases occurred. Radiation monitoring personnel from several government agencies and national laboratories were quickly assembled at TMI-2. The data they collected were analyzed by specialists from the Commonwealth of Pennsylvania; the national laboratories operated by the U.S. Department of Energy; the U.S. Nuclear Regulatory Commission; the U.S. Environmental Protection Agency; and the U.S. Department of Health, Education, and Welfare. These efforts made it possible to develop estimates of the dose received by the maximally exposed individual and the collective dose received by the more than two million people who lived within 80.5 km from the plant. The collective dose estimates ranged from 28 to 35 person-Sv (2800 to 3500 person-rem). The maximum dose to any individual was estimated to be <1 mSv (100 mrem). Essentially all of the off-site collective dose was due to the release of ∼370 PBq (10 million Ci) of noble gases. In addition, <1.1 TBq (30 Ci) of 131I and 0.148 TBq (4 Ci) of 133I are estimated to have been released to the atmosphere, with small amounts of other radionuclides released to the Susquehanna River. When accepted risk coefficients are applied to the estimates of collective dose, it is concluded that no radiation-induced cancers would be expected to occur in the lifetime of the exposed population.