Pseudohomozygous and pseudoheterozygous type II hyperlipoproteinemia.

A 4-year-old boy had typical features of homozygous type II hyperlipoproteinemia with planar xanthomas, a plasma cholesterol level greater than 600 mg/dl, and an estimated beta-lipoprotein cholesterol concentration greater than 500 mg/dl. Both he and his sister, who had the biochemical features of the heterozygous state, responded unusually well to a diet low in saturated fat and cholesterol content. These features, combined with the finding of normocholesterolemic parents, are highly suggestive of a newly described syndrome. pseudohomozygous type II hyperlipoproteinemia.