Objective. To examine the effectiveness of oral vitamin D(3) (400 IU) supplementation on the nutritional vitamin D status of breastfeeding infants. Design. As part of a larger ongoing vitamin D RCT trial of lactating women, infants of mothers assigned to control received 1 drop of 400 IU vitamin D(3)/day starting at one month of age. Infant 25(OH)D levels (mean +/- S.D.) were measured by RIA at visits 1, 4, and 7. Results. The infant mean +/- S.D. 25(OH)D at baseline was 16.0 +/-9.3 ng/mL (range 1.0-40.8; n = 33); 24 (72.7%) had baseline levels <20 ng/mL (consistent with deficiency). The mean levels increased to 43.6 +/-14.1 (range 18.2-69.7) at 4 months and remained relatively unchanged at month 7: 42.5 +/-12.1 ng/mL (range 18.9-67.2). The change in values between 1 and 4 months and 1 and 7 months was statistically significant (P </= .0001), and despite a decrease in dose per kilogram, values were not significantly different between months 4 and 7 (P = .66). Conclusions. Oral vitamin D(3) supplementation as an oil emulsion was associated with significant and sustained increases in 25(OH)D from baseline in fully breastfeeding infants through 7 months.