On August 29, 2005, Hurricane Katrina devastated the Gulf Coast Mississippi region, damaging health care infrastructure and adversely affecting the health of populations left behind. Operation Assist, a project of the Children's Health Fund and the Columbia University Mailman School of Public Health, operated mobile medical units to provide health services to underserved populations in the affected areas. Data collected from all patient encounters from September 5-20, 2005 demonstrate that in addition to common respiratory illnesses, skin conditions, and minor injuries, a high proportion of visits were for vaccine administration and chronic medical problems including hypertension, diabetes, and asthma. Mobile medical units staffed by primary care clinicians experienced in dealing with the clinical and social needs of the underserved and comfortable working in a resource-poor environment can make a positive contribution to post-disaster care.