A large and very well organised study from Denmark has shown that there is a significantly increased risk of infantile hypertrophic pyloric stenosis (IHPS) in infants exposed to macrolide antibiotics. Data were collected over 16 years on nearly one million (999,378) singleton liveborn babies. Overall, 30,091 women (3%) took macrolides during pregnancy and 21,557 (2.2%) after birth. A total of 6,591 infants (0.6%) were given macrolide treatment directly.