The purpose of this study was to determine the maximum tolerated dose and dose-limiting toxicities of fish oil fatty acid capsules containing omega-3 fatty acid ethyl esters. Twenty-two patients with neoplastic disease not amenable to curative therapy who had lost 2% of body weight over a previous 1 month time period were given an escalating dose of fish oil fatty acids. The maximum tolerated dose was found to be 0.3 g/kg per day of this preparation. This means that a 70-kg patient can generally tolerate up to 21 1-g capsules/day containing 13.1 g of eicosapentaenoic acid + docosahexaenoic acid, the two major omega-3 fatty acids. Dose-limiting toxicity was gastrointestinal, mainly diarrhea, and a poorly described toxicity designated as "unable to tolerate in esophagus or stomach." A patient with chronic lymphocytic leukemia taking the fish oil provided an unusual opportunity to perform a detailed biochemical study of the effect of fish oil capsules on the lipids of malignant cells at several sequential time points in treatment. Studies of the malignant lymphocytes, serum, and whole blood of this one patient revealed an increase in eicosapentaenoic acid, the major component of the fish oil capsules, during fish oil capsule treatment. This study provides a scientific basis for the selection of omega-3 fatty acid doses for future studies in cancer. The maximum tolerated dose found is considerably higher than anticipated from published studies of many human diseases. The observation of a modification of the lipids of leukemic cells, serum, and blood in a patient with chronic leukemia provides a biochemical basis for a possible effect of fish oil supplements on cancer cachexia and tumor growth.