After operations of benign gynaecological diseases we recorded changes of the lipid metabolism which were manifested by the following deviations of serum cholesterol and non-esterified and esterified fatty acid concentrations: 1. Total serum cholesterol levels declined markedly after operation either throughout the six-day period after operation, or they began to rise starting on the third day. 2. The esterified fatty acid concentrations rose after a brief drop, usually already from the first day. 3. The non-esterified fatty acid concentrations rose after operation. The peak was as a rule reached on the third day after operation. Subsequently they had a declining trend. 4. The preoperative values of the above lipids were slightly altered (we observed higher cholesterol and esterified fatty acid concentrations). This change was at least partly explained as the consequence of the preoperative stress. In our opinion is it important to attempt to restrict sudden changes of the lipid metabolism, as these changes may become the basis of various postoperative complications.