The objective of the study was to examine the association between lactation and maternal subclinical cardiovascular disease. The Women and Infants Study of Healthy Hearts enrolled 607 mothers who delivered a singleton between 1997 and 2002. In 2007, participating mothers underwent measurements of carotid intima-media thickness, lumen diameter, adventitial diameter, and carotid-femoral pulse wave velocity. Multivariable linear and logistic regressions were used to estimate the associations between lactation and subclinical cardiovascular disease. Compared with mothers who breastfed for 3 or more months after every birth, mothers who never breastfed exhibited a 0.13 mm larger lumen diameter (95% confidence interval, 0.04–0.22) and a 0.12 mm larger adventitial diameter (95% confidence interval, 0.02–0.22) in models adjusting for age, parity, birth outcome, sociodemographic variables, health-related behaviors, family history, gestational weight gain, early adult body mass index, current body mass index, C-reactive protein, blood pressure, cholesterol, triglyceride, high-density lipoprotein, glucose, and insulin levels. Mothers who do not breastfeed have vascular characteristics associated with a greater risk of cardiovascular disease.