In order to establish the reference ranges of lipids and apolipoproteins apoA-I and apoB in serum for different periods of pregnancy we examined 719 healthy pregnant women. 172 of them were in the first trimester, 227 in the second and 320 in the third trimester. The control group, consisting of 65 healthy nonpregnant women, was used to determine the magnitude and statistical significance of the lipid changes. All lipids and apolipoproteins apoA-I and apoB were significantly elevated during the second and the third trimesters. The most prominent change was a 2.7-fold triglyceride increase in the third trimester; in the same trimester the mean level apoB increased by 56%, total cholesterol by 43%, low-density lipoprotein (LDL) cholesterol by 36% and apolipoprotein A-I by 32%. High-density lipoprotein (HDL) cholesterol rose maximally (25%) in the second trimester. The 95th percentiles of the distributions in the second and the third trimesters are: total triglyceride: 2.87 and 4.68 mmol/l; total cholesterol: 8.24 and 9.83 mmol/l; LDL-cholesterol: 5.61 and 6.48 mmol/l; apoA-I: 2.48 and 2.61 g/l; apoB: 1.47 and 1.92 g/l. The fifth percentiles of HDL-cholesterol distributions are: 1.09 in the second trimester and 1.04 in the third one. The ratios of total cholesterol to HDL-cholesterol, of LDL-cholesterol to HDL-cholesterol and apoB to apoA-I were decreased in the first trimester, without change in the second, and increased in the third trimester. These data may indicate that hyperlipidaemia of pregnancy is not atherogenic.(ABSTRACT TRUNCATED AT 250 WORDS)