Long-chain n-3 polyunsaturated fatty acids (n-3 LCPUFA) have been proposed to have beneficial effect on cancer cachexia. The aims of the present study were to a) determine the incorporation of n-3 LCPUFA in erythrocytes (RBC) as a measurement of compliance to fish oil (FO)-supplement in lung cancer patients undergoing anti-neoplastic therapy; and b) evaluate the effect of the FO-supplement on weight-loss, mid arm muscle circumference, energy and protein intake, hand grip strength and quality of life. Forty-two patients with advanced lung cancer were randomized immediately after referral to ingest either 20 ml of FO or 20 ml of rapeseed oil (RO) daily. Patients were evaluated every three weeks. Twenty-five patients participated in the study for more than 21 days. The RBC content in FO-group increased with 35%, 137% and 44%, respectively (p < 0.001, p < 0.001 and p < 0.001), but did not change in the ROg roup. Neither intention-to-treat analysis nor per-protocol-analysis revealed any statistically significant differences between the groups with respect to clinical outcomes.