We assessed the effect of vitamin D supplementation on related biochemistry, infection and dentition of the infant. In a double-blind, placebo-controlled trial conducted in Lucknow, India (latitude 26°N), 230 mother -newborn pairs were randomised to receive, for 9 months, 3000 µg/month oral vitamin D 3 by the mother (group A) or 10 µg/d by the infant (group B) or double placebo (group C). All babies received 15 min of sun exposure (unclothed) during massage. Infants' median 25-hydroxyvitamin D (25(OH)D) was lower in group C (median 45·3; interquartile range (IQR) 22-59·5 nmol/l) than in groups A (median 60·8; IQR 41·3-80·5 nmol/l (P < 0·01)) and B (median 61·3; IQR 41·3-75·3 nmol/l (P < 0·05)) at 3·5 months. Infant 25(OH)D correlated negatively with infant parathyroid hormone (r −0·46, P < 0·01). Elevated alkaline phosphatase (>7.5 µkat/l) was significantly more frequent in group C babies (16 %) than in group A (4 %) or group B (0 %) babies. The number of days with respiratory or diarrhoeal infection by 9 months of age was higher in group C (median 46·5; IQR 14·8-73·3 d) than in group A (median 18·5; IQR 8·8-31·0 d (P < 0·01)) or group B (median 13·0; IQR 7·0-28·5 (P < 0·05)). We conclude that monthly maternal or daily infant supplementation with vitamin D along with sun exposure is superior to sun exposure alone in maintaining normal infant 25(OH)D at 3·5 months, and provide protection from elevated alkaline phosphatase and infectious morbidity.Key words: Vitamin D: Lactation: Infants: Sunshine Hypovitaminosis D is highly prevalent in India despite adequate sunshine. Factors responsible for this include inadequate exposure to sunshine, atmospheric pollution and skin pigmentation. Serious consequences of vitamin D deficiency include cardiac failure and hypocalcaemic seizures (1) . Transplacental transfer and breast milk concentration of vitamin D are low in mothers with poor vitamin D status during pregnancy and lactation (2)(3)(4) . Studies from India have shown 84-96 % of mothers and infants at birth and at 3 months to have serum 25-hydroxyvitamin D (25(OH)D) < 20 ng/ml (50 nmol/l), with winter mean 25(OH)D in women being as low as 5·9 ng/ml (14·75 nmol/l) (2,5) . High-dose (160 µg or 6400 IU/d) vitamin D 3 administered to the lactating mother was found to safely and significantly improve maternal 25(OH)D and mean milk antirachitic activity (6) . Ala-Houhala et al. (7) showed that a regimen of 50 (but not 25) μg/d to the mother improved the nursing infant's serum 25(OH) D significantly. In the only study from India on postnatal supplementation of term infants, Kumar et al. (8) did not find any differences between the placebo and supplemented groups (receiving 35 µg vitamin D 3 /week) in mortality or hospital admissions (their primary outcomes) or referral to the outpatient clinic for moderate morbidity, although serum 25(OH)D was higher in the treated group. Current international recommendations mention 10 µg/d as routine supplementation for breastfeeding infants (3,4,9) . However, no study from tropic...