HOSPITAL admissions for anaphylaxis have trebled in five years at the [Melbourne] Royal Children's Hospital - and no one knows why.

Unpublished data shows that 71 children were admitted to the hospital in 2005-06 after suffering an anaphylactic reaction, mainly to food. Five years earlier there were 23 admissions.

Associate Professor Mimi Tang, the director of the hospital's allergy and immunology department, said about half the referrals to her department were now for food allergies.

"Food allergy and anaphylaxis are rising faster than any other allergic disorder in children," she said. While asthma rates had plateaued, if not fallen, "we are now just at the start of a wave of food allergy and anaphylaxis".

Professor Tang said the rise could be due to increased awareness by parents or the hospital being more cautious in admitting people.

Allergic reactions to food including milk and eggs can include severe facial swelling and rashes, vomiting and, in more serious cases, difficulty breathing or collapsing. Most children grow out of allergies, but 80 per cent don't grow out of peanut allergies.

A separate study published in this week's Medical Journal of Australia suggests the dramatic rise is affecting all hospitals.

An analysis by Associate Professor Raymond Mullins, a clinical immunologist and allergy physician at the John James Medical Centre in Canberra, found that Australian hospital admission rates for food anaphylaxis in children aged up to four grew from 39.3 per million children in 1993-94 to 193.8 per million in 2004-05.

Professor Mullins said the analysis, based on figures by the Australian Institute of Health and Welfare, suggested the growth in food-induced anaphylaxis was greater in that age group than any other.

Hospital admission rates generally for food anaphylaxis rose from 36.2 per million to 80.3 per million people in that period.

Professor Mullins said national studies were needed to investigate the true prevalence of food allergies and guide public health policy.

At his private clinic, the number of children aged up to five treated for food allergies had grown 12-fold in just over a decade - from 11 patients in 1995 to 138 last year.

And the trend is continuing. Professor Mullins said in the first six months of this year, he had already seen 10 per cent more children than at the same time last year.

"It's in epidemic proportions. The problem is we don't know why it's gone up."

Professor Tang said there was a need to train GPs to be able to manage milder cases of food allergies and to invest more into research into prevention.

Katie Allen, a pediatric gastroenterologist and allergist at the Murdoch Children's Research Institute, said people were also likely to be genetically susceptible.