We studied the molecular basis of a case of apolipoprotein CII (apo CII) deficiency with a history of familial consanguinity. DNA sequence analysis of the apo CII gene from the patient revealed a homozygous nucleotide change: a T-->C transition for codon 26 (TGG) at nucleotide 2967 of the third exon resulting in a Trp26-->Arg substitution. His mother was heterozygous of the same mutation and showed half the value of normal apo CII/apo CIII. Analysis of his brother who showed the normal apo CII concentration revealed no mutation at the same place. These results suggested that this missense mutation could be the cause of apo CII deficiency in this kindred.