Background There is still uncertainty about the nature and relative impact of early determinants on childhood blood pressure. This study explored determinants of blood pressure at the age of 6 years in 2 Dutch birth cohorts. Methods and Results Results of hierarchical multiple linear regression analyses in GECKO (Groningen Expert Center for Kids With Obesity) Drenthe study (n=1613) were replicated in ABCD (Amsterdam Born Children and Their Development) study (n=2052). All analyses were adjusted for child's age, sex, height, and body mass index (BMI), and maternal education and subsequently performed in the combined sample. No associations were found between maternal smoking during pregnancy and childhood blood pressure. In the total sample, maternal prepregnancy BMI was positively associated with systolic blood pressure (SBP) (β [95% CI], 0.09 [0.02-0.16] mm Hg) and diastolic blood pressure (β [95% CI], 0.11 [0.04-0.17] mm Hg). Children of women with hypertension had higher SBP (β [95% CI], 0.98 [0.17-1.79] mm Hg). Birth weight standardized for gestational age was inversely associated with SBP (β [95% CI], -6.93 [-9.25 to -4.61] mm Hg) and diastolic blood pressure (β [95% CI], -3.65 [-5.70 to -1.61] mm Hg). Longer gestational age was associated with lower SBP (β [95% CI] per week, -0.25 [-0.42 to -0.08] mm Hg). Breastfeeding for 1 to 3 months was associated with lower SBP (β [95% CI], -0.96 [-1.82 to -0.09] mm Hg) compared with no or <1 month of breastfeeding. Early BMI gain from the age of 2 to 6 years was positively associated with SBP (β [95% CI], 0.41 [0.08-0.74] mm Hg) and diastolic blood pressure (β [95% CI], 0.37 [0.07-0.66] mm Hg), but no effect modification by birth weight was found. Conclusions Higher maternal prepregnancy BMI, maternal hypertension, a relatively lower birth weight for gestational age, shorter gestational age, limited duration of breastfeeding, and more rapid early BMI gain contribute to higher childhood blood pressure at the age of 6 years.