Polyunsaturated fatty acids (PUFAs) play an important role in both induction and prevention of carcinogenic process. It is well known that several types of neoplastic cells show decreased total PUFA content, contributing to their resistance to chemotherapy and lipid peroxidation. In the light of this, human lung cancer A549 cells, with low PUFA content, were exposed to arachidonic or docosahexaenoic acid to investigate the effect of n-6 and n-3 PUFAs on growth and elucidate underlying mechanisms. The bulk of the results showed that both n-6 PUFAs and n-3 PUFAs decrease human lung-tumor cell growth in a concentration-dependent manner, inducing cell death mainly evident at 100microM concentration. The mechanism underlying the antiproliferative effect of n-6 and n-3 PUFAs appeared to be the same, involving changes in fatty acid composition of biomembranes, production of cytostatic aldehydes derived from lipid peroxidation and able to affect DNA-binding activity of AP-1, and induction of PPAR. From these results it may be hypothesized that n-6 PUFAs, like n-3 PUFAs, are able to inhibit tumor growth.