It is important to know the likelihood of medical events that can happen during a spaceflight mission so that the resources needed to treat the event are available. Since there is limited volume, mass, and power on a spacecraft, the medical resources must be optimally chosen so that the resources are there when needed and so that space is not wasted on excess supplies. The NASA Integrated Medical Model (IMM) project is systematically working towards developing an optimal medical kit, using probabilistic risk assessment. The IMM project has identified all of the credible medical conditions that could occur during a spaceflight, and they are working to assign a likelihood of occurrence and a severity of impact for each condition. Decisions on the proper resources to include in the medical kit are made by comparing the risk of each medical event against all of the others. The purpose of this study was to estimate the probability of traumatic chest injury during an International Space Station (ISS) mission. This is a medical event that has a low probability of occurrence but could have a significant impact if it were to happen. Since there has not been an occurrence of traumatic chest injury to date in space, deterministic modeling combined with Monte Carlo simulations were used to estimate the probability of it occurring. The key components of the model were a profile of masses and velocities of equipment that could impact an astronaut. A biomechanical model of the thorax was used to calculate an impact response to the input profile. Normalized compression of the chest was the output response of the biomechanical model and it was correlated to severity of injury. The correlation was used in a logistic regression to determine the probability of injury. The probability of injury was combined with the probability of occurrence of an impact to the chest during one year on the ISS, to determine the overall probability of traumatic chest injury during one year on the ISS. The model parameters were modeled as distributions, so that the variability and uncertainty of each parameter was captured within the probability estimate. Monte Carlo simulations were performed and the probability of traumatic chest injury during one year on ISS was determined to be: 5.32 x 10-4 ± 5.95 x 10-4 (4.16 x 10-5 – 1.39 x 10-3), presented as the most likely probability ± the standard deviation (90th percentile confidence interval). In addition to the probability estimate, results of a sensitivity analysis and a verification and validation analysis of the biomechanical model of the thorax are given. The most sensitive parameters of the model are, in order of sensitivity: 1) velocity of the impactor; 2) rate of occurrence of an impact; 3) The intercept coefficient in the probability of injury equation; 4) mass of the impactor. The output of our biomechanical thorax model matches the output obtained by the original authors of the biomechanical thorax model, which verifies that the biomechanical thorax model was implemented correctly. Additionally, the output response of the biomechanical thorax model falls within the response corridor derived from the results of another cadaver impact study.