The effects of essential fatty acids on cell proliferation and cell viability of 3 human tumor and 4 normal cell lines were tested in vitro. It was found that n-3 and n-6 fatty acids supplemented at 20 micrograms/ml killed human breast, lung, and prostate cancer cells selectively. Normal human fibroblasts and other normal cells were not killed, but their rate of division was lowered. The most selective cytotoxic effects were obtained with fatty acids containing 3, 4, and 5 double bonds. The degree of inhibition of growth and/or loss of viability depended on a given fatty acid, on cell density, on fatty acid concentration, and on the type of cell. When eicosapentaenoate, gammalinolenate, or arachidonate was added to cocultures made of human cancer cells with normal fibroblasts, the cancer cells were selectively eliminated. These results, combined with the observation that certain fatty acids coupled to cytotoxic agents may enhance the cytotoxic activity, suggest that treatment of cancer with polyunsaturated fatty acids containing 3, 4, and 5 double bonds has potential clinical usefulness.