Fatty acid derivatives are abundant in biological membranes, mainly as components of phospholipids and cholesterol esters. Their presence, free or bound to phospholipids, modulates the lipid membrane behavior. The present study shows the differential influence of the C-18 fatty acids (FAs), oleic, elaidic, and stearic acids on the structural properties of phosphatidylethanolamine (PE). X-ray diffraction of PE-FA systems demonstrated that oleic acid (OA) produced important concentration-dependent alterations of the lipid membrane structure: it induced reductions of up to 20-23 degrees C in the lamellar-to-hexagonal transition temperature of 1-palmitoyl-2-oleoyl PE and dielaidoyl PE and regulated the dimensions of the hexagonal lattice. In contrast, elaidic and stearic acids did not markedly alter the phospholipid mesomorphism. The above effects were attributed to the different "molecular shape" of OA (with a kink at the middle of the molecule) with respect to their congeners, elaidic and stearic acids. The effects of free fatty acids (FFAs) on membrane structure are relevant for several reasons: i) some biological membranes contain very high levels of FFAs. ii) Mediterranean diets with high OA intake have been shown to exert protective effects against tumoral and hypertensive pathologies. iii) FFA derivatives have been developed as antitumoral and antihypertensive drugs.