We describe a novel double-labeling method to simultaneously investigate proliferation and apoptosis from plastic-embedded biopsy specimens (PEBs). Infusions of bromo- and/or iododeoxyuridine (BrdU/IudR) were given to 10 patients, five with acute myeloid leukemia (AML) and five with myelodysplastic syndromes (MDS), and S-phase cells were measured in PEBs using a monoclonal anti-IudR/BrdU antibody. Apoptosis was measured by in situ end-labeling (ISEL) of DNA. The results demonstrate that both AML and MDS are highly proliferative disorders but that there is almost no apoptosis in the former, whereas extensive apoptosis was observed in the latter. Double labeling revealed that large numbers of S-phase cells in MDS were simultaneously undergoing apoptosis. We conclude that the high cell death in MDS cancels the high cell birth, resulting in a functionally aplastic marrow and thus accounting for the observed ineffective hematopoiesis. On the other hand, AML is rapidly fatal, probably owing to high cell birth with no or minimal cell death. Therapeutic strategies to prevent intramedullary programmed cell death of hematopoietic precursors should be evaluated in MDS, and efficacy of chemotherapy in AML can be assessed by measuring the induction of apoptosis in post-treatment biopsy specimens.