Apolipoprotein E phenotypes were determined on 417 consecutive lipid clinic patients using an isoelectric focussing technique. Of the 15 patients with phenotype E2/2, 13 (3.1%) had type III hyperlipoproteinaemia and 2 obese identical twins had type V. A further 20 patients (4.8%) had similar plasma and lipoprotein lipid levels but were E2 heterozygotes (14 E3/2 and 6 E4/2). They displayed a widened pre-beta-band almost confluent with the beta-band rather than the broad beta-band shown in classical E2/2 type III patients. In view of the similarities between these heterozygotes and the classical homozygous (E2/2) type III patients and their occurrence in the same families we suggest the nomenclature homozygous and heterozygous type III. In a subsequent comparison between 30 E2/2, 22 E3/2 and 8 E4/2 type III individuals the only significant difference in plasma and lipoprotein lipid parameters was a lower VLDL cholesterol to triglyceride ratio of 0.85 in E3/2 patients than that of 1.24 in E2/2 patients (P less than 0.01). Both homozygous and heterozygous patients showed premature ischaemic heart disease and both responded dramatically and similarly to treatment with clofibrate. These observations indicate that apo E phenotyping is worthwhile in all patients with combined hyperlipidaemia and that homozygous and heterozygous type III hyperlipoproteinaemia is not uncommon.