The effect of maternal diet on serum 1,25-dihydroxyvitamin D has not been determined in human lactation. Serum 1,25-dihydroxyvitamin D, other calcitropic hormones, osteocalcin, and minerals were examined in lactating and nonlactating women consuming a vegetarian or nonvegetarian diet. The vegetarian diet was a macrobiotic diet consisting primarily of whole cereal grains and vegetables; dairy products, eggs, and meats were generally avoided. We tested the thesis that the effect of lactation on serum 1,25-dihydroxyvitamin D is more pronounced in women on vegetarian diets than in those on nonvegetarian diets. Serum 1,25-dihydroxyvitamin D concentrations were significantly higher in lactating women compared with nonlactating women and in vegetarian compared with nonvegetarian women. Among vegetarian women, mean serum 1,25-dihydroxyvitamin D concentrations were 37% higher in the lactating group. For all subjects, serum parathyroid hormone was elevated during lactation compared with nonlactation. Thus, a vegetarian diet appears to be associated with increased serum 1,25-dihydroxyvitamin D during lactation.