Being miserable or stressed will not increase your risk of dying, according to the UK's Million Women Study.
It had been thought that being unhappy was bad for health - particularly for the heart.
But the decade-long analysis, published in the Lancet, said previous studies had just confused cause and effect.
However, experts argued that unhappiness in childhood may still have a lasting impact.
A series of studies had shown that how happy people are, strongly predicts how long they are going to live.