Some clinicians have been reluctant to add human milk fortifier (HMF) to expressed human milk because of concerns about potential feeding intolerance. Based on our own observations, we hypothesized that the addition of HMF to unfortified human milk does not cause feeding intolerance in premature infants. Data from a 5-yr, controlled, prospective study of feeding in premature infants were evaluated. Seventy-six human milk-fed infants (birth weight 1065 ± 163 g, gestation 27 ± 1 wk) who started receiving HMF at 22 ± 7 days of age were evaluated. The following variables were tabulated for the 5 days before and the 5 days after the addition of HMF, with each infant serving as his/her own control (paired t-test): episodes of emesis, abdominal distention, gastric residual volumes (GRV) > 0, ≥ 2 ml/kg, and > 50% of a previous 3-hr feeding, stool frequency/characteristics, apnea and bradycardia, number of hours feedings were withheld, and number/results of abdominal radiographs. There were significant increases in episodes of emesis (183%), GRV ≥ 2 ml/kg (63%), and stool frequency (54%) in the 5-day period after compared with the 5-day period before HMF addition. Milk intake, however, doubled during this interval. When adjusted for differences in volume of milk intake, increases in emesis and GRV were not significant. Furthermore, increases in emesis and GRV did not affect the time when full gavage or oral feedings were achieved. These data suggest that any apparent feeding intolerance after adding HMF to human milk is a reflection of increased volume rather than the result of adding the HMF.