Adverse drug reactions (ADRs) are common problems in both paediatric and adult medicine. The aim of this study was to prospectively identify the ADRs causing hospital admission of children and identification of the risk factors and involved drugs. The study was performed at the University Hospital in Olomouc, Czech Republic. All patients aged 19 years or under admitted to hospital were included in the study, and all admissions for ADRs were prospectively screened for a period of 9 months. Suspected ADRs were subsequently evaluated in detail, and causality assessment was undertaken to determine whether each suspected reaction was possible, probable or definite. The assessment of ADR causality was performed using the Naranjo algorithm, the Liverpool ADR Causality Assessment Tool and the Edwards and Aronson causality assessment method. During the study period, 2903 admissions were identified; of these, there were 143 admissions of patients with an oncological disease. Sixty-four admissions (2.2%) were caused by an ADR. Anticancer chemotherapy accounted for 35% of the cases, followed by antibiotics (18%), immunosuppressants and vaccines (9% each). The use of different scoring systems does not lead to the differences in the numbers of ADR-diagnosed patient but may result in differences in the determination of the level of certainty. ADRs cause a substantial proportion of children's hospital admissions. The majority of the ADR-diagnosed patient affected the hematopoietic and gastrointestinal systems; the drugs most frequently involved were cytotoxic agents and antibiotics. The most important risk factors identified were female sex and oncological disease.