This paper evaluated a method for predicting the incidence of serious adverse events (SAEs) using only clinical trial registration data from ClinicalTrials.gov. Analyzing data from 22,107 two-arm, parallel-group comparative trials, we developed two models: a classification model to predict whether the experimental group would have a higher rate of SAEs than the control group, and a regression model to predict the rate of SAEs in the control group. Pretrained language models, such as ClinicalT5 and BioBERT, were used for feature extraction, and a sliding window technique was used to process long trial descriptions. The optimal model (ClinicalT5+Transformer+MLP) achieved an area under the curve (AUC) of 77.6% for predicting the group with a higher rate of SAEs and an 18.6% root mean square error (RMSE) for predicting the rate of SAEs in the control group. The sliding window technique outperformed the direct comparison method.