OBJECTIVES: This study was performed to investigate the effect of keeping a medical cost diary on medical utilization and medical expenditure in asthma patients. METHODS: Two types of medical utilization survey were conducted to asthma patients receiving regulatory medical treatment from 31 institutions. Firstly, the survey participants were required to remember medical utilization associated with asthma during the last one year using face-to-face questionnaire. Secondly, these patients were asked to keep a medical diary about their daily medical expenditures and symptom associated with asthma for next three months. The patients who keep a diary received regular calls from study staffs not to loose follow-up. All costs were converted to one year cost. A total of 251 asthma patients completed these two studies. RESULTS: Outpatient, pharmacy, inpatient, and oriental medicine costs were significantly reduced after keeping cost a diary; US$470 to $341 in outpatient, US$1,340 to $987 in pharmacy, US$469 to $62 in inpatient cost, and $248 to $35 in oriental medicine, respectively. Emergency visit cost and complementary medicine cost were also reduced, but not significant. The number of absence or early leave due to asthma also decreased after keeping a cost diary. CONCLUSIONS: Generally, keeping a cost diary affected on medical utilization in terms of expenditure as well as disease control. It maybe due to that those patients became cost-conscious or more conscious on their regular treatment by keeping a medical diary. It seems to imply that keeping a medical cost diary can be a good tool for medical expenditure control and disease management.