This longitudinal study on 94 families examined the extent to which parent sensitivity, infant affect, and affect regulation at 4 months predicted mother-infant and father-infant attachment classifications at 1 year. Parent sensitivity was rated from face-to-face interaction episodes; infant affect and regulatory behaviors were rated from mother-infant and father-infant still-face episodes at 4 months. Infants' attachment to mothers and fathers was rated from the Strange Situation at 12 and 13 months. MANOVAs indicated that 4-month parent and infant factors were associated with infant-mother but not infant-father attachment groups. Discriminant Function Analysis further indicated that two functions, "Affect Regulation" and "Maternal Sensitivity," discriminated infant-mother attachment groups; As and B1-B2s showed more affect regulation toward mothers and fathers than B3-B4s and Cs at 4 months, and mothers of both secure groups were more sensitive than mothers of Cs. Finally, the association between maternal sensitivity and infant-mother attachment was partially mediated by infant affect regulation.