Word stress instruction has increasingly become a focus among EFL teachers. Owing to the phonological differences between the learner's first language and English, EFL learners of different nationalities encounter varying degrees of difficulty when pronouncing English polysyllabic words. The majority of EFL students studying medicine tend to find it particularly challenging to pronounce English polysyllabic medical terms correctly. To gain better insights into this issue, this study aims to assess Thai medical students' ability to correctly mark stress in English polysyllabic medical terms and to analyze their errors in identifying their stressed syllables. A purposive sampling was employed to recruit seventy-five mixed-ability first year medical students enrolled in the course entitled "Foundation English for Medical Profession II" of the academic year 2020 in a public university in Thailand to participate in the study. The participants were asked to pronounce 40 medical terms via Zoom, and the quantitative data were analyzed statistically. Nine students of mixed English ability took part in the semi-structured interviews, and thematic analysis was employed to analyze the qualitative data. In light of the key findings, pedagogical implications and recommendations are discussed and highlighted in the study, which found it evident that word stress instruction should be integrated in the EAP/ESP courses for medical students either as part of the core lessons or through supplementary online resources.