A considerable number of patients suffer preventable harm due to diagnostic errors every year. Our knowledge of underlying causes of missed diagnoses, and the types of diagnoses that are often missed, are largely based on autopsy studies and data from malpractice claims, which may over-represent diagnoses that cause death or serious disability. The 1300 pediatricians and pediatric trainees surveyed in this study identified misdiagnosis of viral illnesses as bacterial infections and failure to recognize medication side effects as the most common types of diagnostic error. Faulty information gathering and suboptimal communication were named as the principal individual and system factors leading to diagnostic error. Physicians named closer follow-up and reliable test management systems as major system improvements that could reduce the risk of diagnostic error.