Objective: We aimed to elucidate the accuracy and limitations of 18F-fluoro-L-dihydroxyphenylalanine ([18F] DOPA positron emission tomography (PET) for Japanese patients with congenital hyperinsulinism.Although [18F] DOPA PET is reported to be useful for precisely localising the focal form of congenital hyperinsulinism, previous reports are mostly from European and North American centres.Patients: Seventeen Japanese infants with congenital hyperinsulinism.Measurements: [18F] DOPA PET studies were performed, and the results were assessed by simple inspection or by a quantitative measurement termed the Pancreas Percentage, which expresses the uptake of the head, body, or tail of the pancreas as a percentage of the total maximum standardised uptake value of the whole pancreas.The results were compared with those of other studies, including genetic analysis and histology.Results: By simple inspection, when a single focal uptake was obtained, the localisation and histology were correct in all cases that underwent pancreatectomy.However, the overall results were consistent with the molecular diagnosis and histology in only 7/17 and 6/12 patients, respectively.The inaccuracy of PET studies by inspection was due to elevated background uptake that mimicked a diffuse or multifocal appearance.The accuracy improved substantially using the Pancreas Percentage;it was consistent with the molecular diagnosis and histology in 10/17 and 9/12 patients, respectively.Conclusions: In contrast to the results of previous reports, [18F] DOPA PET appears to be less efficient for diagnosing Japanese patients with congenital hyperinsulinism.However, the diagnostic accuracy is substantially improved when this technique is combined with the Pancreas Percentage.