Brain CT's of 30 patients with early onset probable Alzheimer's disease (according to NINCDS-ADRDA work-group criteria), were compared to those of 30 sex- and age-matched control subjects. This study has confirmed that patients with Alzheimer's disease have higher cerebral atrophy than control subjects, and that cerebral atrophy is correlated with the severity of the disease appreciated with the mini-mental state and the global deterioration scales. It revealed that patients were more different from control subjects for the subjective index and for the third ventricle index. CT has a good predictive value in differentiating groups, but has a poor predictive value in differentiating individuals.