Background: Despite substantial progress toward measles control are making in China, measles outbreaks in immunocompromised population still pose a challenge to interrupt endemic transmission. This study aimed to investigate the features of measles in pediatric hematology and oncology patients and explore the reasons behind the outbreak. Methods: We collected demographic, epidemiological, and clinical data of immunocompromised measles children.All suspected measles cases were laboratory?confirmed based on the presence of measles IgM and/or identification of measles RNA. The clinical data were statistically analyzed by t?test for continuous variables and Fishers exact test for categorical variables. Results: From March 9 to July 25 in 2015, a total of 23 children with malignancies and post hematopoietic stem cell transplantation (post?HSCT) were notified to develop measles in Shanghai. Of these 23 patients with the median age of 5.5 years (range: 11 months–14 years), 20 (87.0%) had received 1–3 doses of measles vaccine previously; all patients had fever with the median fever duration of 8 days;21 (91.3%) had cough; 18 (78.3%) had rash; 13 (56.5%) had Kopliks spot; 13 (56.5%) had complications including pneumonia and acute liver failure; and five (21.7%) vaccinated patients died from severe pneumonia or acute liver failure. Except the first patient, all patients had hospital visits within 7–21 days before measles onset and 20 patients were likely to be exposed to each other. Conclusions: The outcome of measles outbreak in previously vaccinated oncology and post?HSCT pediatric patients during chemotherapy and immunosuppressant medication was severe. Complete loss of protective immunity induced by measles vaccine during chemotherapy was the potential reason. Improved infection control practice was critical for the prevention of measles in malignancy patients and transplant recipients.