The study tested if effects of a planning intervention on fruit and vegetable consumption, physical activity and body weight may be moderated by self-efficacy. Student nurses and midwives (N = 182) were randomly assigned to the planning or the control condition. The intervention included action and coping plans and was followed by two booster sessions. Follow-up took place at two months after last booster session. The intervention affected fruit and vegetable intake. Additionally, body mass index of overweight participants was reduced. Baseline self-efficacy moderated the effects of the intervention: only respondents reporting strong efficacy beliefs improved their diet and exercise.