Younger generations of Australians are gaining weight faster than their parents. Associated health consequences are likely to ensue unless weight gains are prevented; however, it is unclear how to effectively intervene in this population. Electronic databases for health sciences were searched from April to the end of August 2011. Nine studies were included in the review, eight in the meta-analysis, from 771 abstracts reviewed for eligibility criteria: randomized controlled trials of lifestyle interventions, published in English (1980 onward), aimed at preventing weight gain among healthy subjects 18–35 years. Mean body weight change was the primary outcome. The combined weighted mean change in intervention participants was −0.87 kg (95% CI −1.56, −0.18) and in control participants 0.86 kg (95% CI 0.14, 1.57). Post hoc meta-regression analyses revealed evidence-based interventions of 4 months or longer duration were significantly associated with greater weight loss (−1.62 [95% CI −3.21, −0.04], P = 0.045). The small number, short duration and large heterogeneity of trials means the effectiveness of lifestyle intervention for preventing young adult weight gain remains unclear. Future trials conducted over longer periods with larger samples are urgently required to develop effective programmes that will protect against weight gains in future generations.