The effect of omega-3 polyunsaturated fatty acids (PUFA) on the immune system seems to be beneficial. There has been a number of studies concerning the effect of dietary omega-3 PUFA on different immune parameters. The aim of our present study was to investigate the effect of dietary omega-3 PUFA on T-cell subsets and natural killer (NK) cells of patients with solid tumors. We studied 20 patients with solid tumors who received 18 g fish oil/day for 40 consecutive days. We detected a significant increase in T-helper/T-suppressor cell ratio 40 days into omega-3 supplementation, due mainly to a decrease in the number of suppressor T cells. We concluded that dietary omega-3 fatty acids may have a beneficial effect on the already compromised immune system of patients suffering from solid tumors.