A family with hypobetalipoproteinaemia was studied to examine the Lp(a) lipoprotein, lipoprotein cholesterol, and triglyceride composition of the serum lipids. Lp(a) lipoprotein was measured by immunoassay. Serum lipoproteins were separated by ultracentrifugation. Cholesterol and triglycerides were measured using standard enzymatic assays. Serum apolipoprotein B was low and Lp(a) undetectable in the index patient and in her father and son. Separation of the lipoproteins by ultracentrifugation showed a low cholesterol content of serum low density lipoprotein in the affected family members and also a low triglyceride content of high density lipoprotein particles in two affected members. It is concluded that serum lipoprotein cholesterol is altered in hypobetalipoproteinaemia, and family members of index cases have undetectable serum Lp(a) lipoprotein concentrations.