There are limited data on the frequency of foregone health service use in defined populations. Here we describe Thai patterns of health service use, types of health insurance used and reports of foregone health services according to geo-demographic and socioeconomic characteristics. Data on those who considered they had needed but not received health care over the previous year were obtained from a national cohort of 87,134 students from the Sukhothai Thammathirat Open University (STOU). The cohort was enrolled in 2005 and was largely made up of young and middleage adults living throughout Thailand. Among respondents, 21.0% reported use of health services during the past year. Provincial/governmental hospitals (33.4%) were the most attended health facilities in general, followed by private clinics (24.1%) and private hospitals (20.1%). Health centers and community hospitals were sought after in rural areas. The recently available government operated Universal Coverage Scheme (UCS) was popular among the lower income groups (13.6%), especially in rural areas. When asked, 42.1% reported having foregone health service use in the past year. Professionals and office workers frequently reported 'long waiting time' (17.1%) and 'could not get time off work' (13.7%) as reasons, whereas manual workers frequently noted it was 'difficult to travel' (11.6%). This information points to non-financial opportunity cost barriers common to a wide array of Thai adults who need to use health services. This issue is relevant for health and workplace policymakers and managers concerned about equitable access to health services.