Cholesteryl ester storage disease (CESD) and Wolman disease (McKusick 278000) are two distinct autosomal recessive disorders, both attributable to a severe reduction in acid cholesteryl ester hydrolase/lysosomal acid lipase activity (EC 3.1.1.13). We have identified compound heterozygous mutations in a family with two siblings affected with CESD. Molecular genetic analysis revealed two mutations one of which has previously been seen only in Wolman disease. Analysis of these mutations acting in concert provides new insight into the correlation of genotype with phenotype in these allelic disorders.