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The present study examined preschool-age firstborns' adjustment to siblinghood, as indexed by security of firstborn-mother attachment, in a sample of 194 2-Parent families. Security of firstborn attachment decreased significantly after a secondborn's birth, but the size of the decrease was smaller among firstborns under 24 months relative to 2-5-year-olds. Mothers' marital harmony and affective involvement with firstborns predicted firstborn security before and after the baby's birth, whereas mothers' psychiatric symptoms predicted firstborn security only after the birth. Post-hoc analyses of select subgroups revealed that mothers of firstborns with high security scores before the newborn's birth, regardless of whether scores remained high or dropped after the birth, showed higher levels of psychosocial and behavioral functioning than did mothers of firstborns with consistently low security scores at both time points. However, substantial drops in firstborn security after a secondborn's birth were associated with higher maternal psychiatric symptom scores both prior to and following the birth. Results suggest that quality of firstborn adjustment to siblinghood can be predicted from both structural and familial aspects of the firstborn environment.