This longitudinal study examines reciprocal associations between maternal perceptions of paternal involvement and paternal perceptions of themselves as a father and of their relationship with the mother over the first 18 months after the birth of a first child, that is, during the transition to parenthood. Both members of intact heterosexual couples (n ϭ 183) completed self-report questionnaires when their first child was two, five, and 18 months of age. Each assessment period included measures of fathers' perceptions of the importance of their parental identity, their parental self-efficacy, and their marital satisfaction, as well as mothers' perceptions of the quality and quantity of paternal involvement in child care. Results of cross-lag path analyses indicate that fathers' greater parental self-efficacy at two months predicts mothers' perceptions of greater paternal involvement at five months. Conversely, mothers' perceptions of greater paternal involvement at two months predict greater parental self-efficacy and marital satisfaction in fathers at five months. This study highlights the importance of the first few months after the birth of a child for perceptions of fatherhood within the couple. Results suggest that when couples become parents, new mothers and fathers mutually influence their respective perceptions relative to fatherhood.