Roduit et al observed an association between caesarean section and asthma at the age of 8 years in a large group of Dutch children, and attribute the development of asthma partly to the mode of delivery, possibly through a different and delayed pattern of intestinal colonisation of micro-organisms.1 Although this hypothesis is most interesting, in their discussion the differential reasons for caesarean sections were not addressed. As they state themselves, the prevalence of caesarean section in the Netherlands is low and elective caesarean section is rare. Because of this, the Dutch population of children born by caesarean section might be a highly selected group. One of the main reasons a caesarean section is conducted is a disproportion between the pelvic aperture and the fetal head circumference, and a large neonatal head circumference has been reported as a risk factor for asthma,2 for any atopic disorder when corrected for neonatal body weight,3 for hay fever4 and for raised IgE for common inhalation allergens at age 11.5 It would therefore be most informative if the authors could provide additional data on the differences between the neonatal anthropometric data of the children with and without ceasarean section, and on the indications for caesarean sections themselves. This issue is of significant importance and of clinical relevance because, if indeed a causal relationship exists between mode of delivery and development of asthma, this would certainly make an argument against elective caesarean section for non-medical reasons. It would seem that there is currently insufficient evidence to infer a causal relationship, but it certainly seems worthwhile sorting this out.