Background: There is a high prevalence of vitamin D deficiency (VDD) in exclusively breast-fed infants in the absence of appropriate vitamin D supplementation.Objective: To evaluate the efficacy of two doses of maternal vitamin D supplementation on vitamin D levels of mother-infant pairs and to assess its effect on growth parameters (weight, length and head circumference) and bone mass of infants.Study design: Randomized controlled trial.Participants: Lactating mother-infant pairs (n=220).Intervention: Maternal oral vitamin D supplementation in two doses (group 1: 1,20,000 IU/month and group 2: 12,000 IU/month) for 12 months.Main outcomes: Maternal and infant serum 25OHD levels, and infants’ growth and bone mass.Results: There was high prevalence of VDD at baseline in mothers (94%) as well as infants (98.5%), which was reduced to 43.1% in (mothers) and 46.5% in infants after 12 months. Significantly higher median (IQR) serum 25OHD levels (ng/mL) were observed among mothers in group 1 compared to group 2 [46 (17–159) vs 18 (6–64); P<0.01] and in infants [36.5 (15–160) vs 17 (7–32); P<0.01]. No significant association was observed between growth parameters or bone mass and serum 25OHD levels of mother or infant between the two groups. Four mothers (3.6%) and two infants (1.8%) in group I had serum 25OHD>100 ng/mL, but without hypercalciuria or hypercalcemia.Conclusion: Bolus vitamin D supplementation in the dose of 1,20,000 IU/month was more efficacious in improving maternal and infant vitamin D status at 12 months, as compared to 12,000 IU/month.