Lipoprotein typing was undertaken in apparently healthy women (419) and men (103) in whom measurements of fasting total cholesterol, triglycerides, and cholesterol content of the major lipoprotein fractions were made. In the female group (30 to 59 years) there was a significant rise in total cholesterol, low density lipoprotein (LDL) cholesterol, and triglycerides with age. Using total cholesterol and triglyceride levels for classification, type II hyperlipoproteinaemia was present in 1 per cent and type IV in 6-7 per cent. When LDL cholesterol levels were also considered, type II occurred in 9-8 per cent ( 8-4% type IIA and 1-4% type IIB). In the male group ( 40 to 59 years) there was no increase in cholesterol or triglyceride levels with age: II per cent had type IV hyperlipoproteinaemia and none had a type II abnormality. When LDL cholesterol levels were also considered, type II occurred in 3 per cent ( 2% type IIA and 1% IIB). The apparent increase in LDL cholesterol frequency of type IIA abnormality in women could be merely the consequence of arbitrary selection of upper limits of normality for LDL cholesterol, but may signify improved detection of a high risk group.