Background:This study tests a new concept of ‘booster’ if-then plans to enhance the longitudinal effectiveness of implementation intentions to increase fruit and vegetable intake in a young adult population.Methods:Participants (N¼650) completed measures of motivation and behaviour before being randomised to a 3 (intervention format: Passive control vs. active control vs. if-then plan)2(booster intervention: If-then plan at 3 months vs. no if-then plan at 3 months) between-participants design. Follow-up measures were taken at three (T2) and six months (T3). Analyses were conducted on an intention to treat basis, ANOVA was used.Findings:(a) total intake increased by 0.23 portions over 6 months (F(1, 644)¼37.01,p50.01). In the if-then plan conditions: (b) allocating a booster at three months resulted in a 0.53 portion increase from T2–T3 (F(1, 118)¼54.06,p50.01) (c) no booster at three months lead to a decrease by0.22 portions from T2–T3 (F(1, 118)¼15.59,p50.01). Discussion:Booster if-then plans can be used to enhance the longitudinal efficacy of implementation intentions in this area. Further research is required to apply these preliminary ideas to other health behaviours.