Drug-related hospitalisations pose a significant burden to the health-care system. The aim was to investigate the prevalence of drug-related problems (DRPs) and their association with hospital admissions in five internal medicine wards. The study included patients admitted to the nephrology, cardiology, gastroenterology, endocrinology and geriatric ward. Pharmaceutical Care Network Europe classification V9.1 was used for identifying DRPs. In total 535 patients participated in the study. We identified 954 DRPs (range 1-7) in 80.7% of patients. Most DRPs were identified on the endocrinology, cardiology and geriatric ward, and they were associated with the efficacy of treatment (71.4%), adverse drug events (10.2%) and unnecessary drug treatment (18.4%). DRPs were associated with the cause of hospitalisation in 74.4% of patients on the nephrology ward, 60.1% and 60.6% of patients in the cardiology and endocrinology ward, respectively, whereas this number was lower in the geriatric and gastroenterology wards (26.9% and 8.9%, respectively). Suboptimal drug treatment due to medication omissions, was often associated with the potential cause of hospital admission. Focusing on patients with specific diseases and DRPs, rather than reducing the number of medications in primary care, may be potentially rational in the attempt to reduce drug-related hospitalisations.