We propose that, because cross-sex friendships are a historically recent phenomenon, men’s and women’s evolved mating strategies impinge on their friendship experiences. In our first study involving pairs of friends, emerging adult males reported more attraction to their friend than emerging adult females did, regardless of their own or their friend’s current relationship status. In our second study, both emerging and middle-aged adult males and females nominated attraction to their cross-sex friend as a cost more often than as a benefit. Younger females and middle-aged participants who reported more attraction to a current cross-sex friend reported less satisfaction in their current romantic relationship. Our findings implicate attraction in cross-sex friendship as both common and of potential negative consequence for individuals’ long-term mateships.