In some radiation accidents, exposure doses are delivered over days or even months. In all cases the organ system most relevant to a patient's survival is the hematopoietic tissue. There appears to be a threshold of approximately 10 mSv per day above which hematopoietic effects become apparent and hematopoietic failure may occur. Experimental observations in dogs demonstrate that exposure to chronic gamma-irradiation may be tolerated for over a year if the daily dose does not exceed 7 mSv to 18 mSv. The pathophysiological mechanisms are being studied by hematological measures and biomathematical models. The results are in accordance with the assumption of excess cell loss and progressive diminution of the stem cell pool over time until a "turbulence region," with an increased probability of system failure, is approached. Diagnostic procedures require a thorough hematological assessment that includes the stem cell compartment. Therapeutic options include administration of hematopoietic growth factors and stem cell transplantation.