Early nutrition may program obesity and cardiovascular risk later in life, and one of the potential agents is (n-3) long-chain PUFA (LCPUFA). In this study, our objective was to examine whether fish oil (FO) supplementation during lactation affects blood pressure and body composition of children. Danish mothers (n = 122) were randomized to FO [1.5 g/d (n-3) LCPUFA] or olive oil (OO) supplementations during the first 4 mo of lactation. The trial also included a high-fish intake reference group (n = 53). Ninety-eight children were followed-up with blood pressure and anthropometry measurements at 7 y. Diet and physical activity level (PAL) were assessed by 4-d weighed dietary records and ActiReg. The PAL value was 4% lower (P = 0.048) and energy intake (EI) of the boys was 1.1 ± 0.4 MJ/d higher (P = 0.014) in the FO group than in the OO group. Starch intake was 15 ± 6 g/d higher (P = 0.012) in the FO group, but there were no other differences in diet. Body composition did not differ between the randomized groups with or without adjustment for starch intake, EI, and PAL. FO boys had 6 mm Hg higher diastolic and mean arterial blood pressure than OO boys (P <0.01), but girls did not differ. Within the randomized groups, blood pressure was not correlated with maternal RBC (n-3) LCPUFA after the intervention, but PAL values were (r = –0.277; P = 0.038). We previously found higher BMI at 2.5 y in the FO group, but the difference did not persist. The differences in blood pressure, EI, and PAL, particularly among boys, suggest that early (n-3) LCPUFA intake may have adverse effects, which should be investigated in future studies.