The hippocampal formation (HF) is known from pathological and MRI studies to be severely atrophied in established Alzheimer's disease. However, it is unclear when the earliest changes in the HF occur. We performed a longitudinal study of asymptomatic individuals at risk of autosomal dominant familial Alzheimer's disease in order to assess presymptomatic changes in the HF. Seven at risk members of a familial Alzheimer's disease pedigree associated with the amyloid precursor protein 717 valine to glycine mutation underwent serial MR scanning and neuropsychological assessments over 3 years. These assessments were compared with results from 38 normal controls. During the study three at risk subjects became clinically affected. Volumetric measurement of the HF showed that asymmetrical atrophy developed in these subjects before the appearance of symptoms. Verbal and visual memory measures declined in parallel with hippocampal loss. A loss of up to 8% per annum of the volume of the HF occurred in the 2 years over which symptoms first appeared. These findings may have implications for early diagnosis of Alzheimer's disease.