In a search of features that might be relevant to the understanding of the hyperbilirubinaemia of infants with hypertrophic pyloric stenosis (HPS), we examined the duodenal fluid in 11 infants with this condition. Four (36%) had an unconjugated bilirubin level above 2.5 mg/dl in the serum. Levels of electrolytes, bicarbonate, liver function tests and cholesterol were similar in the jaundiced and the non-icteric infants. Examination of duodenal fluid for pH, concentration of bilirubin, bile salts, electrolytes and beta-glucuronidase levels also did not disclose any significant differences between the HPS patients and the controls. Bacterial culture of the fluid yielded similar results in both groups. We may conclude that the unconjugated hyperbilirubinaemia observed in some patients with HPS is not associated with overgrowth of bacteria, changes in glucuronidase levels, pH, electrolytes or biliary obstruction.