Although the World Health Organization-The Uppsala Monitoring Centre (WHO-UMC) system has been suggested as a practical tool for classifying adverse drug reactions (ADRs), verification of such system has not been examined. The objective of this study was to evaluate the usefulness of the WHO-UMC classification for the diagnosis of ADRs. The gold standard was the results of drug challenges and serum tryptase in cases of anaphylaxis. Twenty-seven children had ADRs classified by the WHO-UMC system. The causality terms were 'certain' in 4/27, 'probable' in 6/27, 'possible' in 10/27 and 'unlikely' in 7/27 of the patients. Skin prick tests and intradermal tests were positive in 1/20 and 1/5 of the patients, respectively. Drug challenges and serum tryptase were positive in 8/26 and 1/3 of the patients, respectively. After complete evaluation, the positive and negative ADRs were documented in 9/27 patients (33.33%) and 18/27 patients (66.67%), respectively. The multi-level likelihood ratios for ADRs using the WHO-UMC system were infinity in causality term 'certain', 2 in 'probable', 0.5 in 'possible', and 0 in 'unlikely'. In conclusion, causality term 'certain' and 'unlikely' of the WHO-UMC system had large impact on the likelihood of ADRs. In contrast, the causality term 'probable' and 'possible' had small impact on the likelihood of ADRs. Drug challenges and serum tryptase were helpful to confirm ADRs categorized by WHO-UMC system.