To "make a mountain out of a molehill" or "making a mountain out of a molehill" is an English idiom which means to make a big deal out of a minor issue.

The metaphor is an old one. Horace wrote Parturient montes, nascetur ridiculus mus; "the mountains will labour, a ridiculous mouse will be born." Horace here meant to poke fun at heroic labours producing meager results; his line is also an allusion to one of Æsop's fables, The Mountain in Labor. The title to Shakespeare's play, Much Ado about Nothing, expresses a similar sentiment.

Similar idioms exist in other European languages; a German equivalent is aus einer Mücke einen Elefanten machen, "to make an elephant out of a mosquito". Another idiom of this kind is the Romanian "a face din tantar, armasar", i.e. "to make a stallion out of a mosquito".

The behaviour is frequently exhibited by persons suffering from anxiety disorder.

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