Background: n-3 (omega-3) Long-chain polyunsaturated fatty acids (LC-PUFAs) inhibit fat cell differentiation and fat storage in adults, and this has led to the hypothesis that maternal n-3 LC-PUFA supplementation may reduce fat mass in children. Objective: The objective of this systematic review was to evaluate the effect of n-3 LC-PUFA supplementation in pregnancy or lactation on infant and child body composition in randomized controlled trials. Design: MEDLINE and EMBASE databases were searched for relevant articles. Human trials that supplemented the maternal diet with n-3 LC-PUFAs during pregnancy or lactation and assessed either body fat mass or body mass index in children were included. Trials had to be randomized in design. The quality of all included studies was assessed against set criteria, and results of eligible trials were compared. Results: There were only 3 human trials (4 publications) that met our inclusion criteria. There was considerable disparity in study design and trial quality. The results were variable and showed positive, negative, or neutral effects of maternal n-3 LC-PUFA supplementation on body fat mass in children. Conclusions: This systematic review highlights the paucity of robust data from human studies to evaluate the effect of increased n-3 LCPUFA exposure during the perinatal period on body fat mass in offspring. Further studies are required in which the intervention is confined to the perinatal period and that are sufficiently powered, have appropriate controls, have adequate blinding of participants and investigators, and have high retention rates.