OBJECTIVES: To determine the direct medical cost of treatment in hospitalized patients with adverse drug reactions (ADRs) in regional hospital in the provider’s perspective. METHODS: The direct medical cost were collected from historical profile of the inpatients who were identified using a list of ICD-10 and admitted  in special ward 4/2 in the fiscal year 2009 (From 11/7/2008 - 9/30/2009) in Nakhon Prathom Hospital. They were divided into 2 groups 1) the patients admitted with ADRs, and 2) the patients who admitted and developed ADRs after admission so that extened hospitalization. The direct medical cost including cost of medical care, cost of hospitalization and cost of  laboratory. RESULTS: During study period 34(19.32%) cases of hospitalization patients admitted because of ADRs and 142(80.68%) cases were verified to have ADRs after admission and 137(77.84%) cases were excluded because they developed ADRs but not extended their hospitalization. More than 35% of the patients age between 30 -44 years old in both groups. The 22(56.41%) patients in group 1 did not previously have ADR. Using Naranjo’s Algorithm found that more than 60% of the patients were categorized in probable class. ADR occurred in both groups more than 40% cause by antibiotics. Most frequent ADR were maculopapular(30.77%) and urticaria(15.38%) as well as occurring more than 1 symptoms(15.38%). Total direct medical cost in fiscal year 2010 from patients in both groups was estimated at 185,935.75 Thai Baht , average direct medical cost 4,767.58 Thai Baht per patient. The average direct medical cost in group 1 and group 2 was 5,074.94 Thai Baht and 2,677.55 Thai Baht per person  respectively. CONCLUSIONS: This study emphasized medical and economic impact of the ADRs treatments. ADR surveillance and pharmaceutical care activities would be compared the cost and benefit.