Objectives: A mass casualty incident (MCI) may strain a health care system beyond surge capacity, affecting patterns of care for casualties and other patients. Prior studies of MCIs have assessed clinical care for casualty patients, but have not examined outcomes or expenditures for noncasualty inpatients in the same time period. Methods: This was a retrospective analysis of administrative hospital claims in a state where an MCI with over 200 casualties occurred; two hospitals that admitted casualties of >5% of their inpatient capacity were studied. The surge period was defined as 7 days after the MCI. Using diagnostic codes, patients admitted on the MCI day with diagnoses of burns or inhalation injury were included in the MCI surge cohort. Patients admitted within a time frame of 7 days prior to 7 days after the MCI who were inpatients during the surge period were included in the non-MCI surge cohort. The authors compared the MCI and non-MCI surge cohorts to a mutually exclusive reference cohort (all inpatients during 6 weeks prior to the MCI), regarding key outcomes of hospital length of stay (LOS) and hospital charges adjusted for age, sex, race/ethnicity, and severity of illness. Results: Fifty-five patients met criteria for the MCI surge cohort, 1,369 for the non-MCI surge cohort, and 5,980 for the reference group. Compared with the reference group and adjusted for covariates, the mean (+/- SD) hospital LOS was 4.90 (+/- 1.85) days longer for the MCI surge cohort (95% confidence interval [CI] = 1.67 to 8.84) and 1.34 (+/- 0.16) days longer for the non-MCI surge cohort (95% CI = 1.00 to 1.65). The MCI cohort also had significantly longer mean hospital LOS than the non-MCI surge cohort (difference = 3.56 days; 95% CI = 0.36 to 7.36). Also adjusted for covariates, mean (+/- SD) total hospital charges for the MCI surge cohort were $22,349 (+/-$8,342) greater than for the reference group (95% CI = $8,182 to $39,485). Mean (+/- SD) charges for the non-MCI surge cohort were $4,028 (+/-$633) greater than for the reference group (95% CI = $2,792 to $5,196). The MCI cohort also had higher mean total charges than the non-MCI surge cohort (difference = $18,321; 95% CI = $4,488 to $34,980). Conclusions: When adjusted for severity of illness, casualty patients and noncasualty patients receiving concurrent hospital care have significantly longer LOS and higher charges than typical hospital patients at times unaffected by MCIs. Spillover effects from MCIs for noncasualty patients have not been previously described and have implications for clinical and hospital management in MCI and other high-surge circumstances.