Hyperbaric oxygen therapy is one of new trends of additional treatment, especially for non-di- ving-related diseases in Thailand. Hyperbaric inside attendants have to work under hyperbaric environment to provide medical care for patients in the hyperbaric chamber. This study aims to investigate longitudinal change in lung function in hyperbaric inside attendants (HIAs) and the relationship with hyperbaric exposure. This is a retrospective longitudinal study exploring the adverse long-term effects to the lungs in HIAs. All inside attendants (HIAs) who worked in the public hospitals or medical centres with multiplace hyperbaric chamber in Thailand were included. To be considered for inclusion in the study, inside attendants were required to have at least two follow-up lung function tests and minimum 1-year interval at baseline from annually periodic examination. Lung function of HIAs were compared against reference values of the Thai population. There were 51 subjects with 9.26-year mean period of follow-up. The HIAs showed a significantly decrease in measured lung function in average forced expiratory volume in 1 second (FEV1), forced expi- ratory flow at 25-75% of functional vital capacity (FEF25-75%) and FEV1/FVC ratio over time. The annual reductions in FEV1, FEF25-75% and FEV1/FVC ratio were 22.52 mL per year, 44.92 mL/s per year and 0.48% per year, respectively. The study showed significant differences in annual changes in FVC, FEF25-75% and FEV1/FVC ratio between HIAs and the lung function predicted values for the Thais. However, the results revealed no differences of annual change in FEV1 from predicted values. The average working depths, average session duration and total working hours as HIAs were related with the changes of lung function. Working in a hyperbaric environment does affect the lung function of HIAs. In addition to fitness to work implementation, periodic lung function evaluation should be encouraged to monitor further possible harm to the attendants.