Background: We previously reported that children of mothers who received fish oil supplementation during pregnancy had higher body mass index [BMI (in kg/m²)] at 6 y of age as well as a concomitant increase in fat-, muscle, and bone mass, but no difference in fat percentage. Objectives: Here, we report follow-up at age 10 y including assessment of metabolic health. Methods: This is a follow-up analysis of a randomized clinical trial conducted among 736 pregnant females and their offspring participating in the Copenhagen Prospective Studies on Asthma in Childhood mother-child cohort. The intervention was 2.4 g n–3 (ω–3) Long-Chain PolyUnsaturated Fatty Acid (n–3 LCPUFA) or control daily from pregnancy week 24 until 1 wk after birth. Outcomes were anthropometric measurements, body composition from Bioelectrical Impedance Analysis, blood pressure, concentrations of triglycerides, cholesterol, glucose, and C-peptide from fasting blood samples, and a metabolic syndrome score was calculated. Anthropometric measurements and body composition were prespecified secondary endpoints of the n–3 LCPUFA trial, and others were exploratory. Results: Children in the n–3 LCPUFA group had a higher mean BMI at age 10 year compared to the control group: 17.4 (SD: 2.44) compared with 16.9 (2.28); P = 0.020 and a higher odds ratio of having overweight (odds ratio: 1.53; 95% CI: 1.01, 2.33; P = 0.047). This corresponded to differences in body composition in terms of increased lean mass (0.49 kg; 95% CI: –0.20, 1.14; P = 0.17), fat mass (0.49 kg; 95% CI: –0.03, 1.01; P = 0.06), and fat percent (0.74%; 95% CI: –0.01, 1.49; P = 0.053) compared to the control group. Children in the n–3 LCPUFA group had a higher metabolic syndrome score compared to the control (mean difference: 0.19; 95% CI: –0.02, 0.39; P = 0.053). Conclusions: In this randomized clinical trial, children of mothers receiving n–3 LCPUFA supplementation had increased BMI at age 10 y, increased risk of being overweight, and a tendency of increased fat percentage and higher metabolic syndrome score. These findings suggest potential adverse health effects from n–3 LCPUFA supplementation during pregnancy and need to be replicated in future independent studies. This trial was registered at clinicaltrials.gov as NCT00798226.