Background: Lactation is associated with lower risks for cardiovascular disease in women. Organ-related adiposity, which plays significant roles in the development of cardiometabolic diseases, could help explain this observation. We evaluated the association of lactation duration with visceral (VAT) and pericardial (PAT) fat volumes in women. Methods: Data were obtained from 910 women enrolled in the Coronary Artery Risk Development in Young Adults (CARDIA) study (1985-1986) without diabetes prior to pregnancy who had ≥1 birth during 25 years of follow-up and had VAT and PAT measured from computed tomographic scans in 2010-2011. Cumulative lactation duration across all births since baseline was calculated from self-reports collected at periodic exams. Results: At baseline, the average age of women (48% black, 52% white) was 24 ± 3.7 years. After controlling for baseline age, race, smoking status, body mass index, fasting glucose, family history of diabetes, fat intake, total cholesterol, physical activity, and follow-up covariates (parity, gestational diabetes), the mean fat volumes across categories of lactation [none (n = 221), 1-5 months (n = 306), 6-11 months (n = 210), and ≥12 months (n = 173)] were 122.0, 113.7 105.0, and 110.1 cm3 for VAT and 52.2, 46.7, 44.5, and 43.4 cm3 for PAT, respectively. Changes in body weight from the first post-baseline birth to the end of follow-up mediated 21% and 18% of the associations of lactation with VAT and PAT, respectively. Conclusions: In this prospective study, longer cumulative lactation duration was associated with lower VAT and PAT volumes, with weight gain partially mediating these associations.