The effect of supplemental fluid intake on milk production was evaluated in a randomized, crossover design study of breast-feeding mothers. Twenty-six women whose infants were growing well with breast-feeding alone were enrolled in the study when their infants were 90 to 120 days of age. Twenty-one women consumed at least a 25% increase in fluids (mean 59%, range 26% to 140%). Mean daily milk production was 814 +/- 163 ml/day in the baseline period and 797 +/- 157 ml/day during increased fluid intake. There was no significant change in milk production between study periods and no significant relationship between percent increase in fluid intake and change in milk production nor between volume of fluid intake and volume of milk produced.