Plasma from a patient with fish eye disease has been enriched with autologous high density lipoproteins (HDL) and supplemented with highly purified normal human plasma lecithin:cholesterol acyltransferase (LCAT). Incubation of such plasma at 37 C in vitro resulted in normalization of its low HDL cholesteryl ester percentage, from 23% to 79%, associated with a two-fold increase in both the cholesteryl ester and triglyceride contents of the HDL fraction, as compared to incubation experiments with absent or heat-inactivated purified normal LCAT. The normalization of the HDL cholesteryl ester percentage induced by incubation with purified normal LCAT also was accompanied by an increase in the size of the original fish eye disease HDL particles, which had a mean mass of 115 kd, to HDL particle populations with mean particle masses ranging from 130-220 kd, depending on the concentration of purified LCAT in the incubate. Both HDL cholesterol esterification and particle enlargement were abolished completely by the LCAT inhibitor DTNB and by heat inactivation of the purified normal LCAT. The results give further evidence that fish eye disease is an alpha-LCAT deficiency.