OBJECTIVES: To determine self-care behavior and medication use among Buddhist monks with diabetes mellitus (DM) from the temples in Phasi Charoen district in Bangkok, Thailand.  METHODS: This qualitative study was conducted among all available monks with DM aged 21 years or over staying in the temples in Phasi Charoen district (altogether 28 temples). Data were collected through face-to-face interviews with a semi-structured questionnaire from February to March 2015.  RESULTS: A total of eighteen monks with type 2 DM (median age [range]: 65.1 [42-79] years) were recruited from 12 temples. Most monks took two meals a day, breakfast and lunch (before 12 p.m.). Almost every monk received recommendations on diabetic diet (88.89%), and hence chose DM-appropriate items from the foodstuff offered. However, several of them did not follow diet recommendations believing that they could not choose. Regarding DM knowledge, most monks lacked information on the causes of DM (61.11%), appropriate glycemic level (50.00%) and hypo-/hyperglycemic symptoms (33.33%/61.11%) because the lack of recommendations from healthcare professionals. Regarding exercise, most of them generally exercised by going on an alms-round or performing a monk’s duties (50.00%), whereas 3 monks (16.67%) said exercise was prohibited for  monks. With regard to DM medication, the majority received combination therapy (77.78%) with sulfonylureas and metformin (61.11%). Drug-related problems were detected in 15 monks (83.3%), and the most commonly identified problems were non-compliance (72.22%), especially due to forgetfulness, and adverse effects of medications (27.8%), including hypoglycemia.  CONCLUSIONS: Some DM monks still had inappropriate self-care behavior and medication use, while most of them lacked DM knowledge. Accordingly, healthcare professionals should realize more the importance of providing DM education to the monks for optimal self-care behavior and medication use under the limitations of Buddhist disciplines.