Coronavirus disease 2019 (COVID-19) is spreading worldwide and poses an imminent threat to public health. We encountered 2 cases of COVID-19 with progression resulting in severe respiratory failure and improvement without any specific treatment. To examine the course of infection, we performed reverse-transcription (RT) polymerase chain reaction assay with serum specimens, and serum SARS-CoV-2 RNA was detected in both cases when body temperature increased and respiratory status deteriorated. We, then examined, retrospectively and prospectively, the clinical course during hospitalization by performing serial examinations of serum SARS-CoV-2 RNA status. The findings from our cases suggest that not only is detection of viremia useful as a predictive marker of severity, but also serial serum SARS-CoV-2 RNA results can be helpful for predicting the clinical course.