PIP: Increases in serum triglycerides, cholesterol and pre-beta- and beta-lipoproteins have been considered as significant pathogenic factors regarding the possible thrombotic side effect of treatment with oral contraceptives (OCs) of the combined type containing synthetic estrogen. To investigate the influence of natural estrogen (estradiol-17beta + estriolnorethisterone acetate) vs. synthetic oestrogen (ethinyl estradiol-norethisterone acetate) on serum lipids and lipoproteins in young women compared with independent and individual controls, as controlled doubleblind investigation was performed in 33 women in a total material and 18 women in a selected group. The significant changes were: quantitative increases in serum triglycerides (p , .001) and beta-lipoproteins (p , .01) during treatment with synthetic oestrogen; no such change was observed in subjects on natural oestrogen, which was found to decrease serum cholesterol (p , .05), phospholipids (p , .05), and alpha-lipoprotein (p , .05). Quantitative estimation of serum lipoproteins was found to be more valuable than the relative estimation by electrophoresis. It is concluded that OCs containing natural estrogens induce less significant and, qualitatively, more benign changes in serum lipids and lipoproteins than preparations containing a synthetic estrogen do.