Several influential studies reported sex differences in early care and education (ECE) treatment on young adult IQ and academic outcomes. This paper extends that work by asking whether sex differences in impacts of the Carolina Abecedarian Project emerged during the treatment period or subsequently and whether sex differences were maintained into middle adulthood. The randomized clinical trial (98 % Black, 51 % female) followed 104 infants 5 to 45 years of age. Longitudinal analyses estimated treatment and sex-by-treatment differences at 5 years, from 5 to 21 years, and at 21 and 45 years. Results revealed treated children entered school with higher IQ and reading skills than control children. Treatment impacts on IQ and math increased over time for females and decreased for males yielding sex differences in treatment impacts at 21 and 45 years. These findings suggest that, while the ECE treatment similarly benefited boys and girls in the short term, the long-term impacts likely depended on subsequent experiences.