Segregation studies of Alzheimer's disease (AD) gene and a cloned DNA probe (D21S11), which detects an EcoRI restriction fragment length polymorphism for a sequence located in the medial part of the long arm of chromosome 21, are reported in a large pedigree, in which AD is transmitted as an autosomal dominant mendelian trait. In this pedigree, the AD gene co-segregation with one of the alternative alleles at the probe raises the possibility of using such a marker for presymptomatic diagnosis of individuals at risk for the disease.