In animal studies, n-3 PUFA have been shown to influence body composition and to reduce the accumulation of body fat, thereby affecting body weight homeostasis. In addition, it has been suggested that an additional supply of n-3 PUFA during pregnancy or lactation, or both, would have a beneficial effect on birth weight and infant growth and development. The purpose of the present study was to systematically review interventional clinical trials on the effects of dietary n-3 PUFA supplementation on body weight in adult subjects and in infants whose mothers were supplemented with these fatty acids during pregnancy and/or lactation. A systematic search, focused on n-3 PUFA and body weight, and limited to controlled clinical trials, was performed in different databases. The quality of all included studies was assessed against set criteria, and results of eligible trials were compared. There were few studies targeting this topic. In adults, all of the five studies included, except for one, show no change in body weight by dietary supplementation with n-3 PUFA. Within those trials conducted in pregnant and/or lactating women in which a main outcome was birth weight or growth in infancy, two showed a modest increase in birth weight and the rest showed no effect. None of the trials showed an effect of maternal n-3 PUFA supplementation on infant's weight at the short term. However, it should be noted that a number of limitations, including a variety of experimental designs, type and doses of n-3 PUFA, and high attrition rates, among others, make impossible to draw robust conclusions from this review.