A prospective study about the development of the mandibular second right molar, has been performed with 17 chemotherapy treated children in the Paediatric Oncology department of the Centre Oscar-Lambret (Lille). Our method is based upon the measure of the crown or root length for different periods: periods with or without chemotherapy. These measurements have been performed every 6 months for 18 months with orthopantomograms. We notice for both the crown and the root: a significant difference in growing speed between periods with an without chemotherapy; a speeding up of the mineralization at the end of the treatment rejoining the average figures of a healthy test population.