In three patients with different forms of leukemia, follow-up examinations before, during, and after chemotherapy and bone marrow transplantation were performed by proton chemical shift imaging (1H-CSI). The relative fat and water fractions were computed in representative regions of the marrow in the femur, pelvis, and lumbar spine. On serial examinations the fat fractions increased over time, in agreement with the responses to therapy proven by bone marrow biopsies from the iliac crest. These preliminary results suggest a role for magnetic resonance and CSI in the monitoring of therapy in leukemia and systemic neoplastic diseases.