One challenge many marital couples face is that they experience discrepant levels of sexual desire for one another. Such discrepancies are particularly likely to arise in mixed-sex relationships because, at least in long-term relationships, men tend to have higher levels of sexual desire for their partner than do women. But what underlies this sex difference? We used a dyadic study of 100 mixed-sex community-based newlywed spouses to investigate the role of biological, relational, cognitive, and emotional factors in explaining sex differences in dyadic sexual desire for a long-term partner. Consistent with predictions, wives on average reported lower daily sexual desire for their spouse than did husbands. Moreover, individual differences in men's and women's levels of circulating testosterone explained this sex difference whereas relational (marital satisfaction, commitment), cognitive (sex-role identification, stress, self-esteem), and emotional (mood, depressive symptoms) factors did not. These findings advance our knowledge of factors that influence dyadic sexual desire and may have practical implications for treating relationship distress in mixed-sex marriages.