Asthma can be over-diagnosed, but is more often under-diagnosed. The diagnosis can be made by demonstrating the reversibility of airway obstruction or by a provocation test if the baseline spirometry is normal. The most frequent causes of missing the diagnosis are (1) atypical clinical presentation, (2) misconceptions about age of onset of childhood asthma, and (3) the coexistence of another chronic respiratory illness that may have a more dramatic clinical picture and constitute a "red herring." It is fascinating to speculate on the similarities and differences between classical asthma (which has a usually completely reversible obstruction) and the hyperreactivity and partial reversibility of chronic inflammatory diseases like COPD or CF. Unfortunately much remains to be investigated and little is known at present about these questions. Even in classical asthma, chronic or recurrent inflammation (even by late allergic reaction) could worsen the basic hyperreactivity and this could be amenable to treatment or prevention.