Medical researchers in the US have identified an effective new treatment for autism in an extract from the humble broccoli sprout.

Researchers from the Johns Hopkins School of Medicine conducted a double blind study over 18 weeks of more than 40 young men with moderate to severe autism.

The study found the effect on those treated with the extract was transformative.

One of the lead authors, Professor Paul Talalay, said during the blinding period it became quite obvious to the parents, caretakers and professionals that there were some dramatic changes in behaviour.

"So that I must say we were rather sceptical and somewhat wondered whether there couldn't be anything wrong with the studies," he said.

"And we had all the data professionally examined by an independent firm to make sure that there'd be no systematic error of any sort."

At the moment, other than behavioural modification programs, there is no effective medical treatment for autism.

"Nearly all efforts in autism now - and there are many hundreds - deal with trying to ameliorate the behavioural effects, which are endpoints to be sure.

"But the point is that this was designed to look at the cause, the core mechanisms rather than at the symptoms, and that has not been done before or very rarely done before, let's put it that way," he said.

In a double blind study, neither the test subjects nor the researchers know what treatment a subject is receiving - for example, active or placebo.

Professor Talalay said he recognised the sample size of the study was small.

"You know, the New England Journal [of Medicine] publishes of course hundreds and thousands of individuals in studies. But then you look more closely and they almost have to because the effects for most of the phenomena they examine are rather small.

"So to get any live bodies out of a small effect, you have to have a fairly large number of individuals. So [because] we are getting rather large effects, it doesn't really on retrospect matter that this is a relatively small study," Professor Talalay said.

Johns Hopkins has been looking into different aspects of the study for some years.

"On autism, we started about four years ago, on the efforts to control oxidation and information and the effect of heat. Those have been going on for 25 years at Hopkins but they've never been related to autism," said Professor Talalay.

"And it was when we realised that in autism there are defects which are in fact corrected by a plant chemical that we proceeded to do this clinical study."

Professor Talalay said it was a complicated matter and they were not yet ready to tell people what to do definitively.

"But for the moment I think that the advice to eat a plant-based diet is sound advice," he said.

"Even if we don't know exactly how much (extract) they're getting, they're going to get some and that's bound to be somewhat beneficial."