Little is known on whether the effects of marriage education programs such as the Prevention and Relationship Enhancement Program (PREP) hold across genders or are moderated by risk factors (participants’ parents’ relationship instability, participants’ low level of education, being unmarried, previous relationship counseling, who initiated program participation, having many children). This trial of 889 participants revealed medium to large improvements in relationship satisfaction and communication from baseline to posttest. Only a few tentative moderating factors were identified. Women who were at risk, as indicated by previous relationship counseling, even caught up with others without such risk at the outset. However, men with little higher education did not improve on all measures. Thus, overall, PREP appears applicable for the prevention of intimate relationship problems in a broad range of circumstances. One of the strengths of this study includes the examination of PREP effects in a real-life application across an entire Scandinavian country.