Apolipoprotein E phenotypes of normo- and hyperlipoproteinemia in Japanese were examined by the disc gel isoelectric focusing technique, which was modified according to the method of Kashyap et al. (1981). Apoprotein E isoproteins were clearly separated by this method. Six phenotypes (E2/2, E3/3, E4/4, E2/3, E2/4 and E3/4) were determined in 107 cases of normolipoproteinemia and 75 cases of hyperlipoproteinemia. In normolipoproteinemia, apoE phenotype frequencies were similar to those of the Japanese and Caucasian populations which were reported previously. In hyperlipoproteinemia, a higher frequency of phenotype E2/2 and a lower frequency of E3/3 were observed. The apo E phenotypes of type IIa and IIb were distributed similarly to that of normal subjects. In contrast, only 27.8% of type IV patients had E3/3 phenotype. Among type V patients 64.3% was homozygous or heterozygous for E-4, and only 14.3% was homozygous for E-3 (E3/3). The results suggest that the apolipoprotein E phenotypes are similarly distributed among different human races and the apolipoprotein E phenotypes could be one of the etiological factors associated with some types of hyperlipoproteinemia.