The different treatment modalities used in children with cancer, like chemotherapy, surgical intervention, radiotherapy, bone marrow transplantation, can produce early and late side effects and therefore these patients need follow- up for their entire life. The aim of the paper is the refreshment of these undesired effects and the presentation of the results of different studies made on the patients of the Hematooncology Department of the Pediatric Clinic nr. I from Tg- Mureş in this respect. During the period 2001- 2005 we assessed the neutropenic fever, infections, cardio and hepatotoxicity, neurotoxicity, secondary humoral immunodeficiency and hypoacusia. In conclusion we can say, that neurotoxicity caused by vincristin (Sindovin) became more severe and frequent, we lack the severe forms of cardio and hepatotoxicity but the humoral immune system was frequently damaged, mostly in leukemias than in solid tumors. In the future, more attention must be paid to late effects of treatment, in the hope of longer survival of children with cancer.