Polyunsaturated fatty acids (PUFAs) are known to modulate the immune system in vivo and to inactivate envelope viruses in vitro. Patients with AIDS had low total plasma lipid levels and low levels of a number of individual fatty acids. However, the C20 and C22 essential fatty acids of the n-3 series were selectively and highly significantly reduced. Normalization of these fatty acid levels in AIDS patients may be a worthwhile therapeutic aim.