The loss of neurons, which is thought to occur during senile dementia of the Alzheimer type, could alter the fibre organization of the neocortex. In the present study, the density and course of the fibres were measured in histological sections in 14 cases: all were women over 75 years of age who had been living in a long-stay hospital and had been prospectively assessed by the test score of Blessed et al. (1968). Their intellectual status ranged from nearly normal to severely demented. Samples from the supramarginal and prefrontal gyri were studied after Bodian's impregnation. The course of the fibres in 3-dimensional space was determined by two angles: (a) the angle which characterized their orientation in the plane of section and discriminated between horizontal (tangential), vertical (radial) and oblique fibres and (b) the angle of section with which the fibres were cut. The total density of fibres correlated neither with the test score nor with the density of neurofibrillary tangles or senile plaques. The percentage of obliquely orientated fibres and of fibres with a high sectioning angle increased in the course of the disease. This increase was mainly at the expense of the horizontal fibres in the supramarginal gyrus and could be explained by the presence of a great number of small, tortuous and abnormally orientated fibres (kinked fibres) in the most severely affected cases. The methodology employed in this study could be useful in the investigation of fibre architecture in other diseases involving the cerebral cortex.