For large scale exposures of the human population to ionising radiation, there is a need for cost-effective high throughput assessment of radiation exposure levels from biological samples to allow triage decisions to be made. Here we discuss the usefulness of the DNA damage marker gamma-H2AX for this purpose. Foci of gamma-H2AX form in response to radiation-induced DNA doublestrand breaks and can be quantified by immunofluorescence microscopy or flow cytometry. Several studies have analysed this marker in patients' blood samples to determine radiation exposures during various diagnostic or therapeutic radiation treatments. Such planned exposures involve only a moderate number of samples which can be obtained at a prearranged time following exposure. In contrast, application of this method as a triage tool in large scale radiological emergencies demands high throughput sample processing and analysis. The rapid kinetics of gamma-H2AX induction and loss presents a major challenge to its successful application as a triage tool. These and other as yet unresolved questions are discussed.