Mulitple marrow core biopsy specimens obtained sequentially from each of 15 cases of treated acute leukemia were studied to established the sequence and frequency of histopathological events developing after chemotherapy. The features were separated into two overlapping phases: depletion and reconstitution. The depletion phase was characterized by progressive loss of hematopoietic cells which eventually left an empty stromal network of reticulin fibrils and numerous dilated sinusoids filled with fibrin. During the reconstitution phase, hematopoietic cell proliferation was variously accompanied by fat cell generation, fibroplasia, and bone formation. Fibrin exudation and serous atrophy of fat, two features previously ascribed to the marrow of leukemia after chemotherapy, were neither common nor characteristic.