Aldehyde oxidase (AO) plays an important role in metabolizing many drugs, so AO activity in individual patients may be a useful parameter for dose adjustment to avoid severe toxicity. In this study, we investigated the developmental changes of AO activity in 101 children. Urine was collected in the morning, and AO activity was assessed in terms of the ratio of pyridone formation from N(1)-methylnicotinamide, an AO substrate. Significant correlations were found between AO activity and various growth indices (age, body weight, body surface area, and liver volume). Age showed the moderate correlation (r(2)=0.506). AO activity rapidly increased with increase of the subjects' age up to about 1 year. These findings suggest that the AO activity begins to increase soon after birth. Because AO activity is immature in children below 1 year of age, dose adjustment based on individual AO activity should be made for such patients.