OBJECTIVE To investigate lactating mothers' intake of fat-soluble vitamins in free-living subjects and to what extent cod liver oil supplementation influences the maternal intake in a population with common intake of cod liver oil. The impact of maternal diet on the concentration of fat-soluble vitamins in human milk was studied. METHODS Dietary intake of 77 lactating women was investigated by 24-hour diet recalls and breast-milk samples were taken at the same occasions. Breast milk samples were analyzed for fat-soluble vitamins. RESULTS The median intakes were 927 microg/day for vitamin A, 5.5 mg/day for vitamin E and 3.3 microg/day for vitamin D. Maternal vitamin A, E and D intakes were higher when the diet was supplemented with cod liver oil. Icelandic breast milk was found to have high contents of vitamin A and E. Only vitamin D was too low in breast milk to meet the recommended intake for infants. Retinylpalmitate in relation to lipids correlated with maternal vitamin A intake (r = 0.23, p < 0.05). The group with cod liver oil supplementation had significantly lower levels of gamma-tocopherol in breast milk (p < 0.01), whereas the supplementation did not affect other fat-soluble vitamins. CONCLUSIONS The recommended intake of fat-soluble vitamins for lactating women can more easily be met with a cod liver oil supplementation than diet alone. Only vitamin D in human milk cannot meet the recommended intakes for infants, with normal breastfeeding. There is a relationship between the content of vitamins A and E in human milk and the maternal diet.