The objective of this study was to describe the history of systemic diseases, medication use, drug allergy and cigarette smoking in a group of Thai dental patients at the Faculty of Dentistry, Mahidol University, between July 2003 to February 2008. By systematic random sampling, 1,787 dental chart records were selected to study retrospectively. Data on age, gender, medical and smoking history were collected and analyzed. The results showed that, of these 1,787 dental patients, 581 (32.5%) were men and 1,206 (67.5%) were women. Age range was between 9-80 years with an average of 33.6ฑ10.1 years. Three hundred and ninety-eight (22.3%) dental patients, 110 (6.2%) men and 288 (16.1%) women, had systemic diseases. The most common systemic condition was cardiovascular disease (8.9%), gastrointestinal/liver disease (6.4%), and disease of endocrine system (3.9%), respectively. Prevalence of systemic diseases increased with age. Four hundred and eighty-four (27.1%) dental patients were taking medication. The common types of drug taken were antihypertensive drug (5.8%), analgesic/anti-inflammatory/ antibiotics drugs (5.3%), vitamin and other supplements (4.5%). One hundred and seventy-three (9.7%) dental patients gave history of drug allergy mostly from antibiotics, i.e. penicillin group (3.3%), followed by analgesic/anti-inflammatory drugs (2.2%) and sulfonamides (1.4%), respectively. One hundred and seventy-eight patients (9.9%) were cigarette smoker, with 138 (7.7%) men and 40 (2.2%) women. Men had a significant greater smoking history than women (p = .000). In conclusion, about one fourth of dental patients had history of systemic diseases and taking medication. Smoking and drug allergy were found in one tenth of dental patients. This study suggests that a thorough medical history taking is necessary to provide proper treatment plan and dental care.