Ten patients with Alzheimer-type dementia and nine age-matched normal controls were examined with SPECT, using split-dose 99mTc-labelled exametazime. The baseline condition involved repetition of the word 'yes' or 'no'. The activation condition involved recognition (indicated by a 'yes' or 'no') of words from a previously learned list presented along with distractor words. Patients who performed this task successfully were selected, and efforts were made to match the patients with controls according to their performance on the task, although this was not fully achieved. Uptake of 99mTc-exametazime was estimated at baseline and during the word-recognition task for predetermined regions of interest drawn from a standard neuroanatomical atlas. The baseline task appeared to normalise tracer uptake for frontal, temporal and parietal cortex in the patient group. However, during the recognition task, controls but not patients showed activation effects. These were most prominent in dorsolateral frontal cortex and adjacent anterior cingulate cortex. Among patients, successful performance was correlated with activation of dorsolateral frontal and parietal cortex on the left side. The results confirm the central role of frontal mechanisms in a recognition memory task. The study highlights some of the difficulties of using cognitive challenge tests in clinical groups.