OBJECTIVES: The purpose of this longitudinal descriptive study was to evaluate the influence of interventions by clinical pharmacists on processes, outcomes of care and economical outcomes. METHODS: : The patients who were hospitalized at internal medicine wards, Songkhla hospital during October 1, 2008-September 30, 2009 were recruited to this study. The core set of clinical pharmacy services were admission drug histories, drug protocol management, adverse drug reaction management, drug information, medical rounds and patient discharge counseling. The clinical pharmacists collected patients’ data, pharmacists’ interventions, direct drug cost-saving  and activities on a specific designated form. The data was processed on a computerized database. Clinical pharmacist interventions were counted and classified and analyzed to determine the influence on the quality and cost of pharmacotherapy. Descriptive statistics were used in data analysis. RESULTS: : At the end of the study period the following were found : A total of 3796 patients were recruited. Clinical pharmacy services on internal medicine wards contribute to maximizing the pharmacotherapeutic effect and rationalization of drug therapy in 609 patients (16.0%), increase medication safety in 338 patients (8.9%) and reducing drug expenditures in 575 patients (15.1%). Cost-saving from reconciling process was US$12,682  and cost-saving from pharmacists’ interventions were US$6,298. The frequent type of interventions was maximizing the pharmacotherapeutic effect (40.8%), rationalization of drug therapy (30.3%) and minimizing the risk for developing adverse drug effects (28.9%), respectively. The top two of drug related problems found in cardiovascular drugs (30.5%) and antibiotic drugs (24.8%), respectively.   CONCLUSIONS: The results of this study indicate that inpatient clinical pharmacy services on internal medicine wards contribute to rationalization of drug therapy, increase medication safety and reduced the direct expenditures on medications. Therefore, future studies should include multiple sites such as in a general surgical department and clinical pharmacy services should be continued.