In order to estimate the co-morbidity between ear infections and related childhood diseases, data about the occurrence of recurrent ear infections, tonsillitis, sinusitis and atopic diseases from a population based sample of 7992 Norwegian twins were analysed. Correlational results revealed two general clusters, one consisting of upper respiratory tract infections (URI), the other defined by the atopic diseases. Overall, associations between the diseases were greater in males. The sizes of the correlations within each subgroup of infections were moderate, but significant, ranging from 0.191 to 0.363. Similar results were found for the relationship within the subgroup of atopies, with correlations ranging from 0.134 to 0.466. The correlations between the infectious and atopic diseases were weak. Both ear infections and tonsillitis seemed to be predisposing factors for sinusitis. The relative risk of sinusitis among individuals with a history of ear infections was 3.4 (1.9-6.2) and 1.9 (1.2-3.0) for males and females, respectively. Ear infections conferred an increase in tonsillitis, estimated at 2.3 (1.6-3.0) and 2.0 (1.2-3.6) for males and females, respectively. In conclusion, the present study finds evidence for a common predisposition of upper respiratory infections as well as for atopic diseases, but only moderate correlation between the subgroups. Specifically, between ear infections and hay fever there was no covariation.