OBJECTIVES: This study sought to investigate the impact of the Thai "Health Insurance for People with Citizenship Problems" (HI-PCP) on access to care for stateless patients, compared to Universal Coverage Scheme patients and the uninsured, using inpatient utilization as a proxy for impact. METHODS: Secondary data analysis of inpatient records of Kraburi Hospital, Ranong province, between 2009 (pre-policy) and 2012 (post-policy) was employed. Descriptive statistics and multivariate analysis by difference-in-difference model were performed. RESULTS: The volume of inpatient service utilization by stateless patients expanded after the introduction of the HI-PCP. However, this increase did not appear to stem from the HI-PCP per se. After controlling for key covariates, including patients' characteristics, disease condition, and domicile, there was only a weak positive association between the HI-PCP and utilization. Critical factors contributing significantly to increased utilization were older age, proximity to the hospital, and presence of catastrophic illness. CONCLUSION: A potential explanation for the insignificant impact of the HI-PCP on access to inpatient care of stateless patients is likely to be a lack of awareness of the existence of the scheme among the stateless population and local health staff. This problem is likely to have been accentuated by operational constraints in policy implementation, including the poor performance of local offices in registering stateless people. A key limitation of this study is a lack of data on patients who did not visit the health facility at the first opportunity. Further study of health-seeking behavior of stateless people at the household level is recommended.