A method for the analysis of radionuclide (197Hg chlormerodrin) brain scans is described. Lateral scans have been compared area for area with a mean pattern derived from the scans of clinically normal patients. In making the comparison, allowance has been made for the errors arising both from the finite number of counts recorded in each area and from the patient to patient variation, and the latter has been shown to be acceptably small. Antero-posterior and postero-anterior scans have also been analysed by making a direct comparison of the counts occurring in symmetrically placed 2 × 2 cm areas on opposite sides of the head. Using criteria that give 5 per cent false positive scans, the analysis yielded no false negatives in visually abnormal scans; detected nine abnormalities in 35 scans that had been judged normal by a group of observers, but related to abnormal subjects, and made 23 correct assessments in 30 scans that the observers found equivocal.