The anti-inflammatory effect of systemic corticosteroid (CS) on obstructive lung diseases and seasonal allergic rhinitis is well known. So are the physiological side-effects of CS. A major development in inhalation and biologic treatment as well as hyposensibilisation has taken place in recent years and evidence of personalized use of CS in acute exacerbations increases. We review the side effects of CS and the rationale for use of systemic CS in treatment of obstructive lung diseases as asthma and COPD, as well as seasonal allergic rhinitis.