Acute radiation syndrome is a life-threatening condition that has the potential to affect large populations of humans. Although several animal models of this syndrome are available, the total-body–irradiated mouse has emerged as an important tool to evaluate the efficacy of prospective prophylaxis, mitigation, and treatment compounds. Despite the widespread use of this model, humane endpoints have not been clearly identified. To address this issue, we developed a cageside observation-based scoring system specifically for total-body–irradiated mice to assess the progression of clinical signs associated with acute radiation syndrome. Male C57BL/6 mice (n = 175; age, 8 to 9 wk) received an anticipated LD50 dose of radiation and were observed for progression of clinical signs of acute radiation syndrome for 30 d. All mice were scored individually through cageside observation of their body posture (score, 0 to 3), eye appearance (0 to 3), and activity level (0 to 3). Retrospective analysis of the score data indicated that death could be predicted accurately by using increasing cumulative scores (0 to 9). Total scores of 6, 7, 8, and 9 were associated with mortality rates of 78.6%, 86.4%, 93.3%, and 100%, respectively. Furthermore, scores of 6, 7, and 8 predicted death within 3, 1.5, and 0.5 d, respectively. The use of this scoring system provides investigators and IACUCs with predictive humane, surrogate endpoints for total-body–irradiated mice. This system allows preemptive euthanasia of mice before they become moribund, thereby minimizing pain and distress associated with acute radiation syndrome and improving animal welfare.