On January 12, 2010, a 7.0-magnitude earthquake in Haiti disrupted infrastructure and displaced approximately 2 million persons, causing increased risk for communicable diseases from overcrowding and poor living conditions. Hundreds of nongovernmental organizations (NGOs) established health-care clinics in camps of internally displaced persons (IDPs). To monitor conditions of outbreak potential identified at NGO camp clinics, on February 18, the Haiti Ministry of Public Health and Population (MSPP), the Pan-American Health Organization (PAHO), and CDC implemented the IDP Surveillance System (IDPSS). The Inter-Agency Standing Committee (IASC) "cluster approach" was used to coordinate the Haiti humanitarian response. One of 11 clusters, the Global Health Cluster (GHC), builds global capacity, whereas the country-level cluster (in this case, the Haitian Health Cluster [HHC], led by PAHO) responds locally. During the Haiti response, HHC engaged NGOs serving large camps, established IDPSS, followed trends of reportable conditions, undertook epidemiologic and laboratory investigations, and fostered implementation of control measures. This report describes the design and implementation of IDPSS in the post-earthquake period. The primary challenges to implementing IDPSS were communication difficulties with an ever-changing group of NGO partners and limitations to the utility of IDPSS data because of lack of reliable camp population denominator estimates. The IDPSS experience reinforces the need to improve local communication and coordination strategies. Improving future humanitarian response requires advance development and distribution of easily adaptable standard surveillance tools, development of an interdisciplinary strategy for an early and reliable population census, and development of communication strategies using locally available Internet and cellular networks.