Background The effect of a self-regulatory intervention with its focus on planning sunscreen use is evaluated in comparison to a standard educational condition. Purpose This paper studied whether planning mediates between the experimental conditions and the behavioral outcome. Further, it is examined who benefits more: already motivated or unmotivated individuals. Method College students (N = 253) were randomly assigned to two groups: a self-regulatory and a standard-care condition. Sunscreen use, intention to use sunscreen, and planning were assessed at two points in time, 1 month apart. Results The self-regulatory intervention improved planning to use sunscreen but not the behavior directly. Planning emerged as the mediator between conditions and later sunscreen use, controlling for baseline behavior. Moreover, participants who were less motivated benefited more from the intervention. Conclusions Although it is generally assumed that planning interventions are best designed for already motivated persons, the present findings suggest that less prepared individuals might have more to gain from a brief self-regulatory intervention.