The relationship between frequency of expression and milk output was studied in 25 healthy mothers of premature infants with gestational ages at birth of 28 to 37 weeks. Mothers expressed milk from days 5 to 11 post partum based on card selection, either four or more times per day or three or less times per day using an electric pump. Based on a second card selection, milk expression was either maintained at the same frequency or changed to the opposite frequency for a second week. On days 11 and 18, all milk expressed was quantitated. Of the 25 mothers studied, nine mothers changed expression frequency from four or more times per day to three or less times per day or vice versa, while nine expressed at the same frequency both weeks. These data indicate that frequent expression was associated with a significantly greater milk production (342 +/- 229 mL) than infrequent expression (221 +/- 141 mL). Thus, a low-cost, noninvasive method of enhancing milk production by mothers of nonnursing, prematurely delivered infants was to encourage frequent milk expression.