The occurrence, localization and longitudinal course of non-neoplastic MRI abnormalities in children and adolescents with neurofibromatosis type 1 (NF 1) were studied. Thirty-five patients who satisfied the criteria for NF 1 underwent 114 MRI examinations. They were 9 months to 18 years old at their first examination, and 23 were examined more than once (2 - 11 times). The follow-up time varied from 3 months to 10 years (mean 4 years). Thirty-one patients (89%) showed focal high signal intensities on T2-weighted images in the cerebellum, brain stem, deep cerebral gray matter and, less frequently, in the cerebral white matter. Changes were also seen in 80% and 50% of the proton density-weighted and T1-weighted images, respectively. Newly appearing, growing, decreasing and disappearing lesions occurred contemporaneously and in all ages. New lesions still developed in the late teens. Three lesions showed temporary contrast enhancement. Five expansive lesions were found in four individuals without related clinical symptoms. Four of them receded during follow-up. These cases indicate that the differential diagnosis between neoplastic and non-neoplastic lesions is not clear. The results support the view that high T2-signal lesions are so common in NF 1 that they should be included as another criterion for the diagnosis.