It is best to treat stammering as early as possible, ideally before a child starts school, researchers say.
Stammering can be a real problem at school
About one in 20 children begin to stammer, usually between the ages of two and four.
University of Sydney researchers tested a new treatment - the Lidcombe programme - specifically designed for pre-school children.
Their British Medical Journal study found it was more effective than relying on natural recovery.
Around 80% of children who develop a stammer do recover spontaneously.
As a result, there has been doubt about whether therapy has a positive impact - or is simply being used on children who would recover naturally anyway.
The new study is the first to provide hard evidence that therapy does have a positive effect.
In total 54 children aged three to six took part, of which just over half received the Lidcombe programme.
Each child was diagnosed with a frequency of at least 2% syllables stammered.
After nine months the children who received the Lidcombe programme had reduced their level of stammering by 77%, and 52% had reduced their stammering to 1% of syllables.
In the control group, just 43% had reduced their stammering, and only 15% had achieved the target of stammering on 1% of syllables.
However, the researchers say the programme seems to be less effective once children had reached school age.
They argue that delaying treatment until then risks exposing children to serious social and psychological effects.
"If the disorder persists into the school age years a child is exposed to unacceptable risk of experiencing the disabling effects of chronic and intractable stuttering throughout life."
Norbert Lieckfeldt, chief executive of the British Stammering Association, told the BBC News website that two types of therapy were used to treat stammering.
Family interaction therapy encourages parents to help their child to develop speech in a relaxed, unpressurised atmosphere. It does not address instances of stammering directly. This is still the most commonly used form of treatment in the UK.
In contrast, behavioural therapies, such as the Lidcombe programme, take a more direct approach to the problem, encouraging parents to correct a stammer.
Mr Lieckfeldt said: "The BSA recognises the great significance of this research which demonstrates the benefits of treating pre-school children as early as possible after onset to prevent a lifetime of stammering.
"Parents are still too often confronted with advice by GPs and other like 'ignore it and it will go away'."
The cause of a stammer is unknown, although some experts believe it can be the result of a child's desire to express complex emotions and needs outstripping its capacity to produce the speech.