OBJECTIVES: On December 26th, 2004, a massive earthquake struck Indonesia, triggering a tsunami that affected several countries. This catastrophic event had important implications for health system planning; and thus, underscores the need to study the long-term impact of this disaster. This prospective cohort study aims to identify the determinants of health service utilization in tsunami-affected provinces of Thailand, one and two years after the disaster. The relationship between being affected by the tsunami and health service utilization were also investigated. METHODS: Participants were randomly selected Thai citizens (aged 14+), living in Phuket, Phang Nga, Krabi, or Ranong. Approximately one and two years after the tsunami, participants were interviewed in-person on demographic and socio-economic characteristics, disaster impact, health status, and health service utilization, using five questionnaires. Five types of health services were examined: outpatient, inpatient, home care, medications, and informal/unpaid care. Two-stage sample selection model, more specifically probit model and negative binomial model with Heckman correction, were employed to identify determinants of the probability and intensity of utilization, respectively. RESULTS: There were 2079 participants in the first interview and 1989 in the second interview. Determinants of health service utilization depended on the type of health service being examined, and the assessment time after the disaster. Being affected by the tsunami was significantly associated with the propensity to use medications, two years after the disaster, p=0.0125. CONCLUSIONS: The study findings can assist health providers in identifying population at risk of using health services. Accordingly, regional health officers can request appropriate amount of health resources in order to deliver high quality care tailored to specific population. The findings may also assist policy-makers in the development of long-term disaster recovery plan. Additionally, the study findings may not be specific to a tsunami disaster and may provide insights on post-disaster contexts of other natural disasters.