A rise in the prevalence of childhood asthma has been reported in both the UK and in several overseas countries during recent years. Wide geographical variations have also been demonstrated in the distribution of hay fever, atopic eczema and other allergic symptoms. It is the impression of many parents and some clinicians that allergic symptoms may be even higher amongst children in an island environment compared with a similar population on the mainland. To test this hypothesis, Guernsey and Jersey in the Channel Islands, and the Isle of Man in the Irish Sea participated as an independent arm in the International Study of Asthma and Allergy in Childhood (ISAAC). This entailed self-completion of a 38-part pretested and validated questionnaire in a classroom setting under standardised conditions. A total of 3,772 secondary students in school years eight and nine living in the three islands were compared with 27,507 students of similar age in 93 British secondary schools enrolled in the larger UK study. There are a number of demonstrable differences in climate, home environment and lifestyle factors between the islands and mainland UK. Although results were generally above the UK mean, they were comparable with the reference region of South West England and invariably below the UK maximum. For instance, the prevalence of 'ever wheezed?', 'wheezed in last twelve months?', 'wheezed with exercise?' and 'diagnosis of asthma' were a mean of 50.5%, 34.5%, 31% and 19.7% in the islands compared with a UK prevalence of 48.8%, 33.3%, 28.5% and 20.7%, whilst the prevalence of 'runny nose ever?' and 'runny nose in last twelve months?' was a mean of 46.6% and 36.7% in the islands, compared with 47.1% and 37.9% in the UK mean. These differences were not statistically significant. This information will be reassuring to parents of island children, and also demonstrates that services for the management of asthma and other allergic conditions should not be a higher public health priority in these locations than on the mainland of Britain.