We studied the effect of dietary sodium on BP in a double-blind randomized trial with 245 newborn infants assigned to a normal-sodium diet and 231 to a low-sodium diet during the first six months of life. The sodium intake of the normal-sodium group was almost three times that of the low-sodium group. Systolic BP was measured every month from the first week until the 25th week. At 25 weeks, systolic pressure was 2.1 mm Hg lower in the low-sodium group than in the normal-sodium group. The difference between the groups increased significantly during the first six months of life. These observations are in agreement with the view that sodium intake is causally related to BP level.