The present study was undertaken to assess lipid metabolism in patients with thyroid dysfunction with special reference to serum apolipoprotein levels. Serum lipid, lipoprotein and apolipoprotein levels were determined in 28 hyperthyroid and 16 hypothyroid female patients while untreated and euthyroid. Apolipoproteins were measured by the method of single radial immuno-diffusion (SRID). These results were compared with the values of 28 female controls. In the untreated hyperthyroid group, the serum levels of total cholesterol (TC), high density lipoprotein cholesterol (HDL-C), and low density lipoprotein cholesterol (LDL-C) were significantly decreased compared to the controls and increased after treatment. In hypothyroidism, these values before treatment were higher than those in the controls and decreased after treatment. Serum apo A-I, A-II, B and C-III levels were significantly decreased in the untreated hyperthyroid group compared to the control values. Apo C-II and E levels in hyperthyroidism were identical both before and after treatment compared with the control values, respectively. In the untreated hypothyroidism, apo B, C-II, C-III and E levels were significantly elevated compared to the controls, and these changes in apolipoproteins except apo C-II were restored after treatment. Apo A-I and A-II levels in the untreated hypothyroidism were not statistically different from the values after treatment or those in the control group. Serum thyroid hormone (T3, T4) levels inversely correlated apo B and C-III in all subjects. In hypothyroidism, serum TSH positively correlated with apo B, C-II and C-III. The increase in relative body weight (%RBW) in hyperthyroidism during treatment correlated with the changes of TC and LDL-C. In conclusion, these results indicate that thyroid hormones have a substantial influence on the serum apolipoprotein levels, and that measurement of apolipoproteins as well as lipids and lipoproteins in patients with thyroid dysfunction may be useful to evaluate the lipid metabolism and the effect of therapy.