This article examines a prospective study of more than 1000 patients admitted to a university medical service. The author studied the type and frequency of hospital complications during a given hospitalization and found that nearly 20% of patients experienced a medical complication. Such patients endured longer hospitalizations while suffering from a variety of complications, including developing acquired infections and reactions to diagnostic procedures, therapeutic drugs, or transfusions. The author thoroughly describes the episodes, which range in severity from minor to fatal. This was one of the first studies to document the frequency of iatrogenic injury in hospital care.