Bacteriologic examination of blood, urine, CSF, and gastric aspirate was performed in 88 babies admitted with hypothermia (rectal temperature less than 35 degrees C) during the neonatal period. Infection was common in infants older than 3 days (late-onset hypothermia). In 36 of the 44 (81.8%) infected babies, the gastric aspirate was abnormal, whereas it was abnormal in only three of the 23 (13%) not infected. Infection was much less common in babies younger than 3 days (early-onset hypothermia), and in these the gastric aspirate was mostly normal. Two babies had RDS-like symptoms, and streptococci were cultured from the gastric aspirate. Examination of the gastric aspirate is a quick, efficient method of diagnosing severe infection, not only immediately after birth but throughout the neonatal period.