Older people tend to have multiple comorbid conditions and subsequent polypharmacy, which place them at higher risk of adverse drug events, drug-drug and drug-disease interactions and IP. IP includes several patterns, such as inappropriate dose or duration, prescribing drugs having significant drug-disease or drug-drug interactions, and the omission of potentially beneficial medications. The main purpose was to evaluate the prevalence of IP among medical inpatients in a medical center, so to evaluate the associative factors of IP in Taiwan. From January to December of 2009, all patients aged 65 years and older who were discharged from the medical wards of Taipei Veterans General Hospital were randomly sampled for study (the sampling rate around 1.0%). The IP was evaluated by the STOPP and START criteria. Each medical record was carefully reviewed by physicians who had been trained. Overall, 520 records of elderly medical ward inpatients (mean age = 79.2 ± 6.7 years, 73.8% males) were included for study. In total, 3455 items of medication were prescribed for these 520 patients (mean = 6.6 ± 3.2 items). According to STOPP criteria, 36.2% of the study subjects had at least one potentially inappropriate medication (PIM). The most common PIMs were: (1) medications that may adversely affect those who are prone to falls, e.g., benzodiazepines, neuroleptics and first generation antihistamines (14.2%). (2) Ca-channel blockers with chronic constipation (12.3%). (3) Use of neuroleptic agents (5.6%). (4) Long-term, long-acting benzodiazepines (2.5%). (5) Prolonged use of first generation antihistamines (2.1%). Besides, 218 patients (41.9%) had at least one potentially prescribing omission (PPO). Common PPOs included: (1) statin therapy in diabetes mellitus if coexisting major cardiovascular risk factors present (19.0%). (2) Antiplatelet therapy in diabetes mellitus with co-existing major cardiovascular risk factors (12.5%). (3) Metformin with type 2 diabetes with or without metabolic syndrome (in the absence of renal impairment) (8.7%). (4) Angiotensin converting enzyme inhibitor or angiotensin II receptor blockers with chronic heart failure (7.3%). (5) Aspirin or clopidogrel with a documented history of atherosclerotic coronary, cerebral or peripheral vascular disease in patients with sinus rhythm (7.1%). Logistic regression showed that older age and number of medications were significant risk factors for PIMs. In conclusion, the prevalence of PIMs among elderly medical inpatients in a medical center in Taiwan was 36.2% and PPOs was 41.9%. Further study is needed to clarify the underlying causes of potentially IP to promote better quality of prescribing for older patients.