Serum lipid values were determined in the first degree relatives of 31 children within the framework of a screening programme for cardiovascular-disease risk factors in schoolchildren of either sex (11 to 12 years old). Familial hyperlipoproteinaemia was not present in 5 families. The investigations revealed 12 families with hypercholesterolaemia and 2 with hypertriglyceridaemia. 12 families showed that form of hyperlipoproteinaemia which has been described in the literature as "familial combined" or "multiple type" hyperlipoproteinaemia. Spontaneous variation in serum lipids was observed in 4 out of the 12 above-mentioned children when determinations were repeated. The variability in expression of hyperlipoproteinaemia in the individual case, as well as in families, is one of the reasons for the controversy over the classification of hyperlipoproteinaemias. It is well known that hyperlipoproteinaemia, especially of the familial type, represents a significant risk factor for cardiovascular disease. Thus, it is essential to treat persons with elevated serum cholesterol or triglyceride values and to examine their relatives for hyperlipoproteinaemia.