The long-term bone marrow culture (LTBMC) system provides an in vitro physiological model for the study of stromal cell mediated haemopoiesis in patients with aplastic anaemia. The two aspects of haemopoiesis--stromal and stem cell function--can be analysed separately using a modification of LTBMC with cross-over studies. Patients with aplastic anaemia universally demonstrate defective stem cell function in terms of reduced or absent marrow repopulating ability, reflecting a deficiency of long-term culture initiating cells. Defects in stromal cell function, as assessed by the ability of aplastic anaemia stroma to support normal generation of haemopoietic progenitors, are not common, but may conceal an isolated deficiency of a particular growth factor in some patients due to the overlapping nature of haemopoietic growth factor activities. The stem cell abnormality in aplastic anaemia reflects a deficiency in cell numbers, as well as dysfunction in certain cases. An increased level of apoptosis in aplastic anaemia marrow CD34+ cells exists, and this correlates well with disease severity. LTBMC studies demonstrate that more of the haemopoietic cells are nonviable (apoptotic and dead) compared with normal controls, and this correlates with reduced colony (CFU-GM) generation. An increase in apoptosis among primitive haemopoietic cells may contribute to the stem cell defect in aplastic anaemia. Haemopoietic growth factors such as G-CSF, when given after immunosuppressive therapy such as antilymphocyte globulin and cyclosporin for aplastic anaemia, may act partly by reducing the increased level of apoptosis, resulting in improved stem cell survival.