Medically and economically asthma is an extremely important disease that affects about 4 per cent of the United States population every year. Most cases require treatment and many individuals are forced to limit their activities. Asthma is most prevalent in childhood and among older adults and the disease can begin or remit at any age. There is an unexplained predominance of boys among childhood asthmatics. Studies involving skin tests, bronchial challenges, aerometric sampling, and clinical examination have established allergy as important in the etiology of both adult-onset and childhood-onset asthma. Allergen sensitivity is particularly important in childhood-onset disease. The efficacy of allergy immunotherapy in the treatment of asthma is not well established. Seasonal excerbations of asthma suggest sensitivity to pollens, molds, or dusts but may also occur in individuals with nonallergic asthma and may be related to climatic conditions or outbreaks of respiratory infection. Asthma patients often have a history of allergic rhinitis or eczema and these conditions are also frequent in the families of asthmatics. These atopic diseases usually occur at the same time and allergic rhinitis in children should not be considered a harbinger of asthma. Asthmatic attacks are frequently initiated by viral infections, and asthmatic patients appear to have an increased susceptibility to viruses. Bacterial infections are not frequently associated with asthmatic attacks, and the routine use of antibiotics for such episodes is not warranted. Breast-feeding of infants appears to reduce the incidence of atopic disease and infections. Infants from allergy-prone families should be breastfed for 6 weeks to 6 months, and highly allergic foods, such as eggs, soy, and cow's milk, should probably be excluded from infant and maternal diets during this period. Parental smoking is associated with an increased risk of childhood asthma. Most childhood asthmatics will have complete remission of their disease but this is less likely if the asthma is very severe or accompanied by chronic sinusitis, nasal polyps, or infantile eczema. Remission in childhood may be due to increasing airway caliber with growth or loss of allergen sensitivity. A significant number of adult asthmatics will also have spontaneous remission of their disease. The natural history of asthma should be considered when evaluating claims for therapeutic interventions.