We investigated whether formulae with evening primrose and fish oils raise long chain polyunsaturated fatty acids (LCPUFA) in plasma cholesterol esters (CE), erythrocytes (RBC) and platelets (PLT) to levels encountered in breast-fed infants. Low birthweight infants (< or =2500 g) received LCP1 formula (n = 16; 0.31% 18:3 omega6, 0.17% 20:5 omega3 and 0.20% 22:6 omega3) or LCP2 formula (n = 13; 0.32% 18:3 omega6, 0.34% 20:5 omega3 and 0.43% 22:6 omega3). Fatty acids were measured days 10+/-2, 20+/-3 and 42+/-3. The formulae raised CE, RBC and PLT 20:5 omega3 and 22:6 omega3 dose-dependently (P<0.01), to exceed levels of breast-fed babies (n = 18) day 42 (P<0.05). CE, RBC and PLT 20:3 omega6 was comparable with, and CE, RBC, PLT 20:4 omega6 were below, that of breast-fed infants (P<0.05). Dietary 20:5 omega3 and 22:6 omega3 related with CE, RBC and PLT 20:5 omega3 and 22:6 omega3 (n = 47; P< or =0.01). Dietary 20:5 omega3 and LCPUFA omega3 related inversely with CE, RBC and PLT 20:4 omega6 and LCPUFA omega6 (P< or =0.002). LCP1 and LCP2 fed infants had similar LCPUFA omega6 status day 42. Added 18:3 omega6 does not correct 20:4 omega6 to that of breast-fed infants, but improves 20:3 omega6 status. Fish oil dose-dependently raises 20:5 omega3 and 22:6 omega3, but decreases 20:4 omega6 and other LCPUFA omega6.