Iodine-enriched (IE) eggs are produced by chickens fed a diet containing kelp. These eggs have been reported to reduce plasma cholesterol in humans and experimental animals. The purpose of this study was to determine the effect of the ingestion of one IE egg/day on the plasma lipoprotein cholesterol in borderline and hyperlipidemic individuals ingesting a low-fat diet. One hundred three subjects with entry cholesterol levels greater than 5.17 mmol/L were placed on a low-fat, low-cholesterol diet for 12 weeks. Between weeks 4 and 12, approximately half of the subjects were randomly assigned to either a diet control group (n = 53), or a group who ingested one IE egg/day in addition to this diet (n = 50). Subjects in both the egg group and the diet control group had a significant reduction in total plasma cholesterol (TC) at the end of the study compared with study entry; addition of the egg in the diet did not abolish the TC reduction in the egg group. However, paired comparisons of total and lipoprotein cholesterol levels at the end of the egg intervention period with the end of the initial dietary period demonstrated that the egg group had a significantly greater increase than the diet control group in TC (egg group: 7.2 +/- 1.5% increase; diet controls: 1.5 +/- 0.9% increase; p less than 0.01) and low-density lipoprotein cholesterol (egg group: 9.2 +/- 1.7% increase; diet controls: 3.9 +/- 1.5% increase; p less than 0.01). This effect was most pronounced in subjects with higher initial cholesterol levels and subjects with mixed hyperlipidemia (elevated cholesterol and triglyceride). Results suggest that these particular groups of subjects are most susceptible to cholesterol changes associated with ingestion of IE eggs.