Lipoprotein patterns and cholesteryl ester transfer activity (CETA) were examined in 2 patients with familial hyperalphalipoproteinaemia (FHALP). The proband was a healthy 58-year-old Japanese male who had an HDL cholesterol of 7.83 mmol/l (301 mg/dl). His sister's HDL cholesterol was 4.52 mmol/l (174 mg/dl), which suggested that both were homozygous carriers of FHALP. In both subjects HDL showed a high cholesterol/apo A-I ratio and appeared to be a larger-sized particle than normal HDL on agarose gel chromatography. Two of the proband's children showed higher HDL cholesterol levels (1.74 mmol/l, 2.16 mmol/l) than normal, but another 2 children showed normal levels (1.48 mmol/l, 1.40 mmol/l). However, the ratios of HDL cholesterol to total cholesterol and to apo A-I in all children were higher than normal. These data suggest, but do not prove, that all his children were heterozygotes. Apo B levels in all of the family members studied were lower than normal (47-80 mg/dl). Deceased members of the same family had not died from cardiovascular disease. Cholesteryl-ester transfer activity was studied in both patients. When serum or lipoprotein deficient serum (d greater than 1.21) and [3H]cholesteryl ester labelled HDL3 were incubated in the presence of an LCAT inhibitor, there was no evidence of cholesteryl ester transfer from HDL to VLDL and/or LDL, unlike normal subjects. The deficiency of CETA in these patients with FHALP presumably accounted for the increase in particle size and cholesterol enrichment of HDL.