Magnetic resonance imaging (MRI) and T1 relaxation time measurements of the vertebral bone marrow were performed in 11 children with acute lymphoblastic leukemia (ALL) at diagnosis. Nine of the children were re-examined after chemotherapeutic treatment. The results were compared with histological data from bone marrow biopsies obtained in close association to the MR examinations. Ten age matched children were examined as a control group. A 1.5 Tesla whole body scanner was used for the measurements. The pretreatment T1 relaxation times of the bone marrow were significantly prolonged, compared to the age matched controls. After chemotherapy the T1 relaxation times of the children with ALL decreased significantly towards or into the normal range. A significant correlation was found between the T1 relaxation time and the content of malignant blast cells in the bone marrow.