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Although exclusively breastfed infants are at increased risk of vitamin D (vit D) deficiency if vit D supplementation is lacking and sun exposure is limited, assessment of both risk factors in the first year of life is lacking. We evaluated the contribution of vit D intake and sunlight exposure to vit D status in 120 healthy, breastfeeding mother–infant dyads, who were followed up for 1 year. Vitamin D intake and skin sunlight exposure were evaluated using questionnaires. Serum 25‐hydroxyvitamin D, parathyroid hormone (PTH) and alkaline phosphatase levels were determined post‐natally in mothers at 4 weeks and in infants at 4, 26 and 52 weeks. Vitamin D supplementation was low (<20%) and sunlight exposure was common (93%) in study infants. At 4 weeks, 17% of mothers were vit D deficient (<50 nmol L⁻¹) and 49% were insufficient (50–<75 nmol L⁻¹), while 18% of infants were severely vit D deficient (<25 …