Hospital-based monitoring is one of the methods used to collect data about drug prescriptions and adverse events. The aim of this 20-day observational prospective study was to evaluate the frequency and type of adverse reaction to antibiotics, and predisposing risk factors in inpatients in six departments of a university hospital (ophthalmology, paediatrics, internal medicine, general surgery, infectious diseases, anaesthesiology and intensive care). The data on all inpatients undergoing antibiotic treatment were collected by physicians trained by our team and validated by an expert panel. Data were recorded on pre-formatted confidential cards (MIO-card). In the 171 inpatients evaluated (125 adults: 39.5% male, mean age 61.6 years, range 21-93; and 46 children: 50% male; mean age 4.75 years, range 3 months-12 years), cefazolin (19.9%), chloramphenicol (18.6%), ceftriaxone (15.4%) and netilmicin (12.9%) were the most frequently used antibiotics. Adverse events occurred in four adults and three children: one had leucopenia (trimethoprim/sulfamethoxazole), one nephrotoxicity (netilmicin+teicoplanin) and one nephrotoxicity (cefotaxime), one diarrhoea (ceftriaxone), one neurotoxicity (isoniazid), one angioneurotic oedema (piperacillin) and one skin rashes (ceftriaxone). A number of strategies (educative and persuasive, facilitative and restrictive) have been proposed to improve antibiotic use. Our study suggests that hospital-based monitoring is a good method with which to detect links between drug exposure and adverse drug reactions in children and adults.