OBJECTIVES: To evaluate the risk for infantile hypertrophic pyloric stenosis 
(IHPS) among infants prescribed systemic erythromycin, infants prescribed a 
course of erythromycin ophthalmic ointment, and infants whose mothers were 
prescribed a macrolide antibiotic during pregnancy.
STUDY DESIGN: Retrospective cohort study of infants born at an urban hospital 
from June 1993 through December 1999.
RESULTS: Of 14,876 eligible infants, 43 (0.29%) developed IHPS. Infants 
prescribed systemic erythromycin had increased risk of IHPS, with the highest 
risk in the first 2 weeks of age (relative risk = 10.51 for erythromycin in 
first 2 weeks, 95% CI 4.48, 24.66). Erythromycin ophthalmic ointment for 
conjunctivitis was not associated with increased risk of IHPS. Maternal 
macrolide antibiotics within 10 weeks of delivery may have been associated with 
higher risk of IHPS but the data were not conclusive.
CONCLUSIONS: This study confirms an association between systemic erythromycin in 
infants and subsequent IHPS, with the highest risk in the first 2 weeks of age. 
No association was found with erythromycin ophthalmic ointment. A possible 
association with maternal macrolide therapy in late pregnancy requires further 
study. Systemic erythromycin should be used with prudence in early infancy.