119 MR-examinations of both tibiae, knees and the lower part of both femur were performed in 41 children suffering from bone marrow disease (27 ALL, 4 AML, 3 NHL, 1 agranulocytosis, 6 anaemia). T1- and T2-spin-echo sequences and a T2-gradient-echo sequence were used. Bone marrow changes in leukaemia were diffuse before therapy and patchy after therapy. Due to their different signal in T2-weighted images, differentiation of the post-therapeutic patchy findings into infiltrations, fibrosis, necrosis and siderosis seems to be possible. In future, MRI will be the method of choice for screening and controlling bone marrow disease if the examination time is shortened by using only a T1-spin-echo sequence and a T2-gradient-echo sequence.