This supplementary appendix provides additional information about the clinical characteristics of coronavirus disease 2019 (COVID-19) in China, as reported by Guan et al. in the New England Journal of Medicine. It includes definitions of exposure and clinical complications, methods for assessing exposure, and laboratory testing procedures. The appendix also outlines the criteria for diagnosing pneumonia, shock, acute respiratory distress syndrome (ARDS), acute kidney injury, secondary bacterial or fungal infections, acute heart failure, and rhabdomyolysis. The study documents the history of contact with wildlife through patient self-reporting and reports the prevalence of Wuhan residents, recent travelers to Wuhan, and those who have had contact with people from Wuhan among patients outside Wuhan. Laboratory confirmation of SARS-CoV-2 was achieved through reverse-transcription polymerase chain reaction (RT-PCR) assays, following protocols established by the World Health Organization. The appendix includes tables and figures that provide detailed clinical characteristics and radiographic manifestations of COVID-19 cases.This supplementary appendix provides additional information about the clinical characteristics of coronavirus disease 2019 (COVID-19) in China, as reported by Guan et al. in the New England Journal of Medicine. It includes definitions of exposure and clinical complications, methods for assessing exposure, and laboratory testing procedures. The appendix also outlines the criteria for diagnosing pneumonia, shock, acute respiratory distress syndrome (ARDS), acute kidney injury, secondary bacterial or fungal infections, acute heart failure, and rhabdomyolysis. The study documents the history of contact with wildlife through patient self-reporting and reports the prevalence of Wuhan residents, recent travelers to Wuhan, and those who have had contact with people from Wuhan among patients outside Wuhan. Laboratory confirmation of SARS-CoV-2 was achieved through reverse-transcription polymerase chain reaction (RT-PCR) assays, following protocols established by the World Health Organization. The appendix includes tables and figures that provide detailed clinical characteristics and radiographic manifestations of COVID-19 cases.