A thermal-hydraulic analysis of the initial 174 min of the Three Mile Island Unit 2 (TMI-2) accident was performed using the THALES (Thermal-Hydraulic Analysis of Loss-of-Coolant, Emergency Core Cooling and Severe Core Damage)-PM1/TMI code. The purpose of the analysis was to verify whether the THALESPMl/TMI code is capable of describing an accident progression in an actual plant. The initial and boundary conditions were based on the TMI-2 Standard Problem data base that was used by the Organization for Economic Cooperation and Development/Nuclear Energy Agency Committee on the Safety of Nuclear Installations in performing the TMI-2 Analysis Exercise. The analytical results generally agree with the actual behavior, indicating that the physical models employed in the code are reasonable. Better results were obtained using this analysis concerning the core degradation behavior in the early phase of the transient in which the debris node was assumed to remain at the original location. However, the physical models for the fuel relocation and debris formation need further improvement to be consistent with accident progression in the later phases of the transient.