Documentation, analysis, and prevention of the harmful effects of armed conflict on populations are established public health priorities [1–5]. Although public health research on war is increasingly framed in human rights terms [6–13], general public health methods are typically applied without direct links to laws of war. Laws of war are international humanitarian laws and customary standards regarding the treatment of civilians and combatants, mainly described in the four Geneva Conventions of 1949 and their Additional Protocols I and II regarding international and civil conflicts [14]. With notable exceptions [11,15–17], absolute numbers are usually reported (e.g., number of persons killed), without systematic description of the proportional effects of armed conflict, thereby limiting the utility of findings and scope of interpretation. In this paper, we introduce the “Dirty War Index” (DWI): a data-driven public health tool based on laws of war that systematically identifies rates of particularly undesirable or prohibited, i.e., “dirty,” war outcomes inflicted on populations during armed conflict (e.g., civilian death, child injury, or torture). DWIs are explicitly linked to international humanitarian law to make public health outcomes directly relevant to prevention, monitoring, and humanitarian intervention for the moderation of war's effects. After choosing the particular outcome to be measured, a DWI is calculated as: Summary Points War, a major public health problem, is a situation where the interests of public health, human rights, and humanitarian law intersect. The DWI is a data-driven public health tool that identifies rates of particularly undesirable or prohibited, i.e., “dirty,” outcomes inflicted on populations during war (e.g., civilian death, child injury, or torture). A DWI is calculated as: (Number of “dirty,” i.e., undesirable or prohibited cases/Total number of cases) × 100. DWIs are designed for direct, easy translation of war's public health outcomes into the human rights, policy, and interdisciplinary work needed to address war's practice. DWIs support monitoring, deterrence, and humanitarian intervention by explicit links to international humanitarian laws and by exposing rates of unacceptable combat outcomes (DWI values) from different weapons or combatant groups. For example: In Table 1, we measure the DWI ratio of “Number of civilians killed/Total number of civilians and opponent combatants killed” using a casualty dataset for Colombia's civil conflict [18]. Table 2 links this DWI to relevant laws of war. DWI values of 99 for illegal paramilitaries, 46 for guerrillas, and 45 for government forces show that paramilitaries are “dirtiest” in terms of proportion of civilians constituting their victims of unopposed attacks (chi-square = 5,010, degree of freedom [df] = 2, p < 0.001). 99% of paramilitary victims were civilians and only 1% were military opponents. This finding, combined with the paramilitaries' methods (execution by close-range gunfire in massacres), suggests intentional targeting of civilians that requires recognition in Colombia's paramilitary demobilization, disarmament, and reintegration process [19]. Table 1 Dirty War Index for Attacks by Actors in the Colombian Civil Conflict, 1988–2005: Civilian Versus Opponent Combatant Mortality Table 2 DWIs Suggested for Measuring Rates of Undesirable or Prohibited Outcomes from Aggression in Armed Conflict As ratios, DWIs complement absolute numbers and lend themselves to comparisons over time, between wars, between weapons, and between warring combatant groups to identify better versus worse performers. Noncombatant wounded-to-killed ratios can provide evidence of war crimes [16]. Proportional “atrocity statistics” [20] from a Darfur survey substantiated US Secretary of State Colin Powell's declaration of genocide and the referral of Darfur's situation to the International Criminal Court [20,21]. By facilitating clear, systematic comparisons, DWIs can help analyze and expose how combatants engage in war and affect populations, thereby increasing the accountability of military and political leaders. This paper describes the theoretical basis and practical applications of the DWI, with brief examples from armed conflicts. More detailed DWI analyses of specific conflicts are planned for future papers.