Axial fast FLAIR images of the brains of 40 normal volunteers in four age groups between 16 and 55 years were examined and the number and size of areas of increased white-matter signal recorded. Increased signal in the corticospinal tract region was seen at the level of the internal capsule in all subjects, extending up towards the centrum semiovale and down towards the pons for 0.5-5.5 cm (median 2.5 cm). In all cases the IIIrd and IVth ventricles were outlined by a thin line of high signal. Focal areas of high signal (caps) were seen around the frontal and occipital horns in 90% and 77% respectively; 54% of caps were asymmetrical. None of the above features varied with the age or sex of the subject, but the numbers of discrete white matter 'lesions' increased with age. The findings are used to suggest guidelines for the identification of areas of 'normal' high signal to be excluded in quantification of lesions on fast FLAIR images.