Numerous published studies have reported that the prevalence rates of asthma and allergic rhinitis have increased over the last two to three decades. When interpreting prevalence data, it is important to consider that reported figures may be biased by differences in classification and diagnostic methods, and increased awareness of the conditions. Studies indicate that asthma and allergic conditions are most prevalent in the UK, Australia and New Zealand. High rates have also been reported for Chile. In Europe, intermediate prevalence rates are seen in southern Europe, and the lowest rates occur in central Europe. Despite factors that may affect the figures, worldwide data provide compelling evidence that the prevalences of both asthma and allergic rhinitis have increased. Prevalence rates and increases in prevalence appear to be greatest in children and young adults. It is estimated that, worldwide, between 1% and 20% of children and young adults have asthma, and around 20% of individuals (all age groups) have allergic rhinitis. In Northern Sweden, data from recent studies indicate that major risk factors for the development of asthma are family history (all age groups), own smoking (adults) or the mother smoking (children), and factors related to house dampness (children). Several studies provide evidence that sensitization to allergens in early life are important risk factors for both asthma and allergic rhinitis. Outdoor air pollution, climatic and 'life-style' factors may also play a role in the development of these conditions. Finally, data from northern Sweden demonstrate the association between asthma and allergic rhinitis, and the results show that an adult with a family history of asthma or rhinitis has a risk of three- to fourfold for developing asthma and of two to sixfold for developing rhinitis over adults without a family history.