• We measured breath hydrogen excretion in 103 neonates from birth to as late as 2 months of age. The patients weighed less than 2000 g at birth and were part of a study of hydrogen excretion as a screening test for necrotizing enterocolitis. Hydrogen excretion in parts per million was normalized for the quality of the expired air by dividing by the Pco2 of the gas sample. The rise in the H2/CO2 ratio was influenced by gestational age, energy intake, and antibiotic usage but not by the daily frequency of feeding. The mean ± SD peak H2/CO2 ratio was 5.1 ± 3.6 ppm per millimeter of mercury and occurred at 16.0 ± 11.0 days of age. The age at which the peak H2/CO2 occurred varied with gestational age. Patients born between 23 and 28 weeks gestational age (n 34) were 22.9 ±13.1 days of age when they experienced their peak H2/CO2 ratio, whereas those born between 29 and 34 weeks gestational age (n = 62) were 12.2 ± 7.5 days of age. The age at which the peak H2/CO2 ratio occurred did not differ between these two groups when corrected for the age at which oral intake exceeded 420 kJ/kg per day. These results suggest that premature neonates require experience with ingesting more than 420 kJ/kg per day before bacteria and carbohydrates are present in large enough quantities to permit measurable hydrogen production. This information will be useful in future studies of premature gut development and physiology and in studying pathologic processes in which malabsorption may play a role.