The purpose of this study was to (a) determine the frequency of diagnostic errors in pediatric cancer, (b) categorize errors, and (c) underscore themes associated with misdiagnosis. This is a retrospective cohort study at a tertiary children’s hospital of 265 patients with new oncologic diagnoses. The diagnostic error rate was 28%. Compared with those with no diagnostic error, those in whom there was an error were more likely to have (a) more visits before diagnosis (P < .001), (b) not been seen in an acute care setting (P = .03), (c) inappropriate treatment (P < .001), and (d) misinterpreted laboratory studies or imaging (P < .001). Themes in diagnostic errors were lack of appropriate evaluation for persistent symptoms (47%), failure to recognize signs and symptoms suggestive of malignancy (45%), and misinterpretation of tests (8%). Clinicians should consider diagnostic evaluation for multiple visits for the same complaint or a constellation of signs and symptoms suggestive of malignancy.