Dietary intake of 18: 2n-6 and 18: 3n-3 may affect endogenous production and incorporation of n-3 long-chain PUFA (LCPUFA) from fish oils (FO). This double-blinded controlled 2 £ 2-factorial 8-week intervention investigates the effects of high and low 18: 2n-6 intake in combination with FO-supplementation on tissue fatty acid composition. Healthy young men (n 64) were randomized to capsules with FO or olive oil (control) (44 (20-56) ml/d) and to either sunflower oil and margarine (S/B) or rapeseed oil and a butter spread (R/K) to provide a high or a low 18: 2n-6 intake. Diet was measured by 4-d weighed dietary records at baseline, during and 8 weeks after the intervention and tissue ncorporation as fatty acid composition of peripheral blood mononuclear cells (PBMC). The fat intervention gave a mean difference in the 18: 2n-6 intake of 73 g/d (95% CI 46, 100) and a similar 18: 3n-3 intake in the groups. The R/K groups had a 02% fatty acid (FA%) (95% CI 00, 04, P¼002) higher content of 22: 5n-3 in the PBMC, a tendency of slightly higher 20: 5n-3 (P¼006), but no more 22: 6n-3 (P¼083) than the S/B groups. FO effectively raised the PBMC content of all n-3 LCPUFA (P,0001). The fat intervention did not markedly influence the effect of FO; the mean PBMC content of n-3 LCPUFA was 103 (SEM 03) FA% in the FO þ S/B group and 106 (SEM 02) FA% in the FO þ R/K group. In conclusion, increasing the 18: 2n-6 intake did not have any pronounced effect on incorporation of n-3 LCPUFA in PBMC, either alone or with simultaneous FO supplementation.