There is limited information available on the vitamin K intake of lactating mothers, concentration of vitamin K1 in breast milk, and the effect of long-term vitamin K1 supplementation of lactating mothers on the vitamin K1 concentration in breast milk. In a randomized study, we followed 20 mothers who received a daily oral vitamin K1 supplement (average 88 micrograms, supplemented group) and 16 mothers receiving no supplement (control group) from 4 throughout 91 days postpartum. Maternal vitamin K intakes (weighed dietary intake) at 4-6, 25-29 and 87-91 days postpartum ranged between 73 and 1735 micrograms/day. Differences between the groups were statistically not significant. Average intake exceeded the recommended dietary intake for lactating women of 55 micrograms/day by 670%. In the supplemented group, mean breast-milk vitamin K1 concentrations (HPLC) at 5, 26 and 88 days postpartum were 1.73 (SD 0.74), 1.36 (SD 0.81) and 1.67 (SD 2.01) ng/ml, respectively. Corresponding values in the control group were 1.44 (SD 0.57), 1.68 (SD 0.70) and 1.78 (SD 1.05) ng/ml. The latter were not statistically different from values in the supplemented group. Mean daily vitamin K1 intakes of infants breast-fed by supplemented mothers were 0.69 (SD 0.42), 0.93 (SD 0.51) and 1.25 (1.53) micrograms, respectively on days 5, 26 and 88. Corresponding values in the control group were 0.69 (SD 0.30), 1.07 (SD 0.58) and 1.31 (SD 0.95) micrograms and were statistically not different from values in the supplemented group. Average vitamin K1 intakes corresponded to 7-13% of the recommended dietary intake of 10 micrograms/d for infants.(ABSTRACT TRUNCATED AT 250 WORDS)