Background & Aim: Until the last decades the chance of survival for patients suffered from different types of malignancies was less. In fact prior to 1950, patients with acute lymphoblastic leukemia had a hope to live less than 20 weeks, but with introducing the new methods of chemotherapy, radiotherapy and surgery, a vast improvement obtained in their prognosis with increase of children's life length suffering from cancer, different specialists concentrated on affects may result from the late treatment. Immediate effects of chemotherapy and radiotherapy on soft tissues had been identified well, but there is a less information from their long term effects on dental tissues during their developments.Material & Method: The purpose of this study which was performed in 1999 was to evaluate the relationship between chemotherapy and enamel DEFECTS in children suffered from malignant diseases who seek medical advise from Mofid children's hospital in Tehran.Results: In this study 42 patients containing, 15 girls and 27 boys -whom were under chemotherapy, were chosen and the same number of healthy control group were chosen assording to the following rules: For omitting the fluorosis factor, all cases were selected from the citizens of Tehran.- In all children with malignancies, chemotherapy should have been finished (In off therapy stage).- Non of children should have been under radiotherapy on dental and jaw area. - From 1471 teeth in 84 patients, 729 teeth (49.6%) were in case group and 742 teeth (50.4%) in control group. - From this number in first group 29 teeth (4%) and 13 teeth (2%) in control group had enamel involvement.- There was a significant relationship between involved teeth in control group (P<0.025).- The result of this study showed, although enamel DEFECTS were more in case group related to control, but this value was not considerable according to total number of children and the observed teeth need more investigation in the future for understanding the probable effects of chemotherapy on enamel genesis.