This study was carried out to disentangle the independent relations of birth order and sibship size with the presence of asthma, allergy and eczema. In a retrospective study, 700 families in the Netherlands were selected with index children born in 1988-90. Data were extracted from reports of health examinations at the age of 6 years of these children and their siblings. Birth order, and not sibship size, appeared to be a strong risk factor for allergy (excluding eczema). Children with higher birth order had a lower risk of allergy compared with first-borns (adjusted odds ratios: 0.43, 0.26 and 0.05 for second-, third- and fourth- or higher borns, respectively; p < 0.0001). Allergy including eczema also had a significant relation with birth order (p = 0.01). For asthma there appeared no clear relation with birth order. For asthma a non-significant relationship with sibship size (adjusted for birth order) was found (p = 0.06): first-born children in small sibships were more at risk than those in larger sibships. For allergy and eczema no such trend was observed. In conclusion, birth order is inversely related to the risk of allergy, independent of the size of the sibship.