Individual variations in serum lipid concentrations resulting from laboratory error are often much less than those due to other factors. Thus the coefficient of variation for biological fluctuations in cholesterol is 5-10% and for triglyceride concentration 5-25%. Many of the causes of changing lipid levels are known. Sampling conditions (venous stasis, body position) contribute as does the handling of the blood sample before it reaches the laboratory, for example whether an anticoagulant is used and haemolysis, clearly meals have a major effect on serum triglycerides, but even when fasting there is probably also diurnal variation. Fluctuations in serum lipids occur during the menstrual cycle, cholesterol and triglycerides tending to be highest at midcycle and to decline towards menstruation. Pregnancy produces major increases in both cholesterol and triglyceride levels. There is a well established seasonal variation in cholesterol concentrations which are greatest in autumn and lowest in late spring and early summer. This may be partly due to nutritional changes, but similar findings in laboratory animals suggest that humoral factors may also be important. Age also influences serum lipids. It is frequently not appreciated that both cholesterol and triglycerides are at their lowest in adolescence. Rapid increases in cholesterol may occur in women around the menopause. Lipid concentrations may undergo considerable change during illness. The decrease in LDL and HDL cholesterol and apo B and tendency for increases in triglycerides during acute myocardial infarction are well known but similar alterations can also occur with angina or coronary artery bypass surgery. Other surgical operations and intercurrent illnesses, perhaps even common viral infections, may also influence lipids.(ABSTRACT TRUNCATED AT 250 WORDS)