Asthma is recognised as a heterogeneous disease and, within this heterogeneity, brittle asthma is identified as a distinct phenotype. Two types of brittle asthma have been recognised in adults: type 1, showing more than 40% diurnal variability in PEF rate on most days over long periods and type 2, with acute severe attacks on a background of apparently good asthma control. The exact incidence of these phenotypes in adults is unknown but both seem to carry an increased risk of death and considerable morbidity. Adults with brittle asthma are generally atopic, with a high degree of psychosocial morbidity. This phenotype has not been recognised to the same degree in children, although we believe that such children exist and probably present the same management problems as their adult counterparts. The management of brittle asthma is, as for all patients with complex disease, holistic, with the removal of important causal factors ensuring compliance and providing adequate education. Steroid therapy remains the mainstay of treatment but the balance has to be struck between the benefits and side-effects of both inhaled and oral corticosteroids. Continuous subcutaneous terbutaline has been shown to be effective in controlling PEF variability but there are side effects and it must be used with due caution.