Fish eye disease (FED) is characterized by severe corneal opacities, causing impaired vision, and dyslipoproteinaemia: hypertriglyceridaemia, raised levels of very low density lipoproteins (VLDL), triglyceride enrichment of low density lipoproteins (LDL) and reduction of high density lipoproteins (HDL). The disease is described in two unrelated families. In both there was a high proportion of low HDL in relatives without eye disease. VLDL, LDL and HDL had normal electrophoretic mobilities. The concentrations of VLDL cholesterol and triglycerides were increased fivefold. LDL cholesterol levels were normal but LDL triglycerides markedly increased. HDL cholesterol was reduced by 90% as were the levels of HDL apolipoproteins. The major part of HDL cholesterol was in the HDL3 fraction. FED HDL were smaller than normal with molecular weights of 115,000 daltons. Lecithin: cholesterol acyltransferase activity and amount of cholesterol esters in serum were normal. Postheparin lipoprotein and hepatic lipases showed normal or subnormal values. Clinically FED differs from other familial conditions with deficiency of HDL such as Tangier disease, LCAT-deficiency and Milano-AI-apoprotein disease. In spite of the extremely low HDL cholesterol FED is not characterized by premature atherosclerosis. Mechanisms for the dyslipoproteinaemia are discussed.