Ten patients with chronic rheumatic diseases were treated either with sunflower oil (linoleic acid 66%; n = 6) or with olive oil (linoleic acid 4%; n = 4) for 21 days. Sunflower oil but not olive oil increased the serum concentrations of linoleic acid in all fractions studied. In cholesteryl esters, both arachidonic acid and dihomo-gamma-linolenic acid concentrations were slightly diminished. The changes in all these fatty acids were already seen on the first days of treatment. Plasma arachidonic acid metabolites showed no uniform changes during the treatment. Excretions of the main metabolite of prostacyclin (6-keto-PGF1 alpha) and thromboxane B2 into urine were slightly increased in most patients on sunflower oil. No marked improvement was seen in the clinical or conventional laboratory parameters in either treatment.