Background: Increasing evidence indicates that adult body composition is associated with prenatal and infancy weight gain, but the relative importance of different time periods has not been elucidated. Objective: The objective was to study the association between prenatal, early postnatal, and late infancy weight gain and body mass index (BMI), fat mass, and fat distribution in young adulthood. Design: We included 403 men and women aged 19 y from a Dutch national prospective follow-up study who were born at <32 wk of gestation. BMI, waist circumference, and waist-to-hip ratio SD scores and subscapular-to-triceps ratio, percentage body fat, fat mass, and fat-free mass at age 19 y were studied in relation to birth weight SD scores, weight gain from preterm birth until 3 mo postterm (early postnatal weight gain), and weight gain from 3 mo until 1 y postterm (late infancy weight gain). Results: Birth weight SD scores were positively associated with weight, height, BMI SD scores, and fat-free mass at age 19 y but not with fat mass, percentage body fat, or fat distribution. Early postnatal and late infancy weight gain were positively associated with adult height, weight, BMI, waist circumference SD scores, fat mass, fat-free mass, and percentage body fat but not with waist-to-hip ratio SD scores or subscapular-to-triceps ratio. Conclusions: In infants born very preterm, weight gain before 32 wk of gestation is positively associated with adult body size but not with body composition and fat distribution. More early postnatal and, to a lesser extent, late infancy weight gain are associated with higher BMI SD scores and percentage body fat and more abdominal fat at age 19 y. © 2005 American Society for Clinical Nutrition.