The epidemiology of Acinetobacter baumannii emerging in combat casualties is poorly understood. We analyzed 65 (54 nonreplicate) Acinetobacter isolates from 48 patients (46 hospitalized and 2 outpatient trainees entering the military) from October 2004 to October 2005 for genotypic similarities, time-space relatedness, and antibiotic susceptibility. Clinical and surveillance cultures were compared by amplified fragment length polymorphism (AFLP) genomic fingerprinting to each other and to strains of a reference database. Antibiotic susceptibility was determined, and multiplex PCR was performed for OXA-23-like, -24-like, -51-like, and -58-like carbapenemases. Records were reviewed for overlapping hospital stays of the most frequent genotypes, and risk ratios were calculated for any association of genotype with severity of Acute Physiology and Chronic Health Evaluation II (APACHE II) score or injury severity score (ISS) and previous antibiotic use. Nineteen genotypes were identified; two predominated, one consistent with an emerging novel international clone and the other unique to our database. Both predominant genotypes were carbapenem resistant, were present at another hospital before patients' admission to our facility, and were associated with higher APACHE II scores, higher ISSs, and previous carbapenem antibiotics in comparison with other genotypes. One predominated in wound and respiratory isolates, and the other predominated in wound and skin surveillance samples. Several other genotypes were identified as European clones I to III. Acinetobacter genotypes from recruits upon entry to the military, unlike those in hospitalized patients, did not include carbapenem-resistant genotypes. Acinetobacter species isolated from battlefield casualties are diverse, including genotypes belonging to European clones I to III. Two carbapenem-resistant genotypes were epidemic, one of which appeared to belong to a novel international clone.Acinetobacter baumannii infections are well-described complications of severe combat-related injuries suffered in Iraq or Afghanistan by military service members (42). Acinetobacter species are notorious for multiantibiotic resistance and survival in the hospital, making eradication difficult or impossible (6, 27). Origin and transmission mechanisms both in wounded service members and in other nosocomial settings remain ambiguous, however. Presumptions of an environmental source (12) are based on literature prior to 1986 before any species differentiation within the genus and are probably incorrect. Hypotheses of Acinetobacter acquisition from the battlefield environment or skin colonization prior to injury are not supported by cultures of healthy service members' skin, nor wound cultures taken within 48 h of injury, nor soil cultures around field hospitals (13,30,41).A. baumannii strains differ in antimicrobial susceptibility and transmissibility. Several reports have shown that multidrug-resistant (MDR) Acinetobacter strains were frequently associated with numerous outbreaks throughout ...