A 52-year-old Japanese woman was evaluated for severe hypertriglyceridemia and recurrent acute pancreatitis. This hypertriglyceridemia was found to be due to the absence of serum apolipoprotein C-II (apo C-II) which was identified by Western blotting using polyclonal anti-apo C-II antiserum. DNA sequence analysis of the apo C-II gene from the patient revealed a homozygous nucleotide change: a thymidine (T) to cytosine (C) substitution in codon 26 (TGG->CGG) at the third exon of the apo C-II gene, that resulted in a Trp26 to Arg substitution. The mutation was also confirmed by restriction fragment length polymorphism (RFLP) analysis with the restriction enzyme Hpa II. The same mutation has been found in a case previously reported in Japan, and was named apo C-II Wakayama. However, the case in Wakayama prefecture showed two concomitant point mutations at the 5'-flanking region upstream from the first exon, which were not identified in our case by RFLP analysis with the restriction enzyme BstXI. Considering that the prefectures of these two cases, Nara and Wakayama, are next to each other, the mutation in our case may be a genetic forebear of apo C-II Wakayama. However, no familial relationship between the two cases has been documented.