Small premature infants are often hypochlorhydric, and frequently their stomachs are colonized by enteric, gram-negative bacteria. We tested a hypothesis that gastric pH affected the colonization of the stomach with enteric bacteria and that this colonization was causally related to the risk or severity of necrotizing enterocolitis. A prospective, double-blind study was conducted that compared a group of infants supplemented with 0.01-0.02 ml of 1 N HCl/ml of milk to a group with a similar supplement of water. Gastric pH, gastric enteric bacteria counts, and the incidence and severity of necrotizing enterocolitis were monitored. The median gastric pH of the HCl-supplemented group was lower (3.0) than controls (4.0) throughout the study (p less than 0.001). The gastric enteric bacterial colonization rate and the quantitative bacterial counts were strongly correlated with gastric pH over 4 (p less than 0.001). Somatic growth rates in infants in the HCl-supplemented group were equal to, or exceeded, those in the control group. There was 1 case of necrotizing enterocolitis among the 34 infants in the HCl-supplemented group and 8 cases among the 34 in the control group (p = 0.02). It appears that acidifying the feedings of small premature infants to a pH low enough to inhibit bacterial proliferation in the stomach significantly lowers the risk of necrotizing enterocolitis.