The purpose of this work is to remind doctors of practical medicine about the importance of differential diagnosis with cancers in general pediatric practice and to increase the vigilance of clinicians regarding the similarity of manifestations of oncology and infectious diseases on the example of specific clinical cases. We conducted an empirical, descriptive study of 4 clinical cases of cancers in children. We selected them among 21 patients who were diagnosed with cancer during the inpatient treatment at the Kyiv City Children’s Infectious Disease Hospital in 2014–2019. The first clinical case describes a brain neoplasm that was initially thought to be acute VZV encephalitis in an 11-year-old child. The second is a case of acute leukemia in a 3-year-old child who was hospitalized with suspected meningococcal infection. In the third case, we presented a 7-year-old patient who was admitted with infectious mononucleosis and as a result, was diagnosed with hemophagocytic syndrome, immunodeficiency and Duncan disease. The latest case demonstrates the combination of tuberculous meningitis and cerebral astrocytoma in a 15-year-old patient, which was initially mistaken for viral encephalitis. Conclusions. Awareness of cancers is important in the diagnosis of infectious nosologies. Dynamic monitoring and the use of a wide range of laboratory and instrumental methods allows to prevent diagnostic errors and to seek treatment in a timely manner.