To confirm the significance of the serum alanine aminotransferase (ALT) test for the diagnosis of fatty liver and to clarify the relationship between serum ALT activity and the duration of obesity, we analysed 310 obese young school children (195M, 115F), who were classified into three duration groups (1-3 y, 4-6 y, 7 + y), three age groups (6-7 y, 8-9 y, 10-11 y), and four obesity groups (weight excess: mild, 20-29%; moderate, 30-39%; severe, 40-49%; very severe, 50%). Seventy-seven patients with abnormal ALT test, > 30 IU/l, and 27 patients with normal ALT test were examined by ultrasound study to identify the fatty-fibrotic pattern of the liver. Abnormal results of the serum ALT test were found in 24% of all patients. The fatty-fibrotic pattern was identified in 64/77 (83%) patients with abnormal ALT test and in 5/27 (18%) patients with normal ALT test. The serum ALT test has a sensitivity of 0.92 for detecting the fatty-fibrotic pattern proven by ultrasound study. Frequencies of cases with abnormal serum ALT levels increased with the duration of obesity. In the shortest duration group, however, the frequencies of abnormal results in serum ALT test did not increase with advanced ages or the grades of obesity. In conclusion, the present study confirmed the usefulness of the serum ALT test for screening fatty liver, and showed that a longer duration of obesity is generally associated with the occurrence of fatty liver in a paediatric obese population. In young patients with mild obesity or a short duration of obesity, however, fatty liver or fatty fibrosis may develop. Early intervention should be made in the case of obese children.