BACKGROUND: The genes encoding Delta(5)- and Delta(6)-desaturases (FADS1 FADS2 gene cluster) were reported to be associated with n-3 (omega-3) and n-6 (omega-6) fatty acid proportions in human plasma, tissues, and milk. Docosahexaenoic acid (DHA) can be supplied especially by dietary fish or fish oil and synthesized from alpha-linolenic acid through a pathway involving these desaturases. OBJECTIVE: We evaluated whether FADS gene variants modify the effect of maternal fish and fish-oil intake on plasma and milk DHA proportions. DESIGN: FADS1 rs174561, FADS2 rs174575, and intergenic rs3834458 single nucleotide polymorphisms were genotyped in 309 women from the KOALA Birth Cohort Study in The Netherlands. Plasma was collected at 36 wk of pregnancy, and milk was collected at 1 mo postpartum. Fish and fish-oil intake was assessed by using a food-frequency questionnaire at 34 wk of pregnancy and updated for the week of milk collection. Gene-diet interactions were tested by linear regression analysis. RESULTS: DHA proportions were lower in women homozygous for the minor allele than in women who were homozygous for the major allele (DHA proportions in plasma phospholipids: P <0.01; DHA proportions in milk: P <0.05). Fish intake ranged from 0 to 2.5 portions of fatty fish/wk, and 12 women took fish-oil supplements during pregnancy. DHA proportions in plasma phospholipids increased with increasing fish and fish-oil intake, irrespective of the genotype. DHA proportions in milk increased only with fish and fish-oil intake in the major-allele carriers. CONCLUSION: Lower proportions of DHA in milk from women who were homozygous for the minor allele could not be compensated for by increasing fish and fish-oil intake, possibly because of limited incorporation into milk.