Aim: Long-chain polyunsaturated fatty acid (LCPUFA) supplementation of infant formula may have a beneficial effect on cognitive development. This study aimed to investigate the effect of LCPUFA formula supplementation primarily on cognition and secondarily on behaviour at age 9 years. Special attention was paid to the potentially modifying effect of maternal smoking during pregnancy. Method: A double-blind, randomized control study was performed in two groups of healthy infants born at term: one group, constituting the control group, received standard formula (n = 169) and another group received standard formula supplemented with LCPUFAs (n = 146). A breastfed group (n = 159) served as an additional reference. At 9 years of age, 72% of the children (control group: n = 123; 71 males, 52 females; LCPUFA group: n = 91; 42 males, 49 females; breastfed group: n = 127, 64 males, 63 females) underwent extensive cognitive and behavioural testing. Results: An interaction between infant nutrition and smoking during pregnancy was found. Among children exposed to smoking during pregnancy, LCPUFA supplementation was associated with higher mean verbal IQ scores (p = 0.007) and learning and memory (p = 0.006). Among children not exposed to smoking during pregnancy, LCPUFA supplementation was associated with lower mean verbal memory scores (p = 0.003). Executive function scores were significantly lower in the LCPUFA-supplemented group than in the control group (p = 0.001). Breastfeeding was associated with better performance on IQ (p = 0.005). Interpretation: No consistent beneficial effect of LCPUFA formula supplementation on cognitive development in term-born infants was found. The study confirmed that breastfeeding is associated with better cognition. (Contains 2 tables and 1 figure.)