THE number of cot deaths in Scotland rose 16% last year, reversing a long-standing downward trend, according to figures released yesterday.

Deaths peaked at 142 in 1989, and were already in freefall before campaigns by broadcaster Anne Diamond and the Government to encourage parents to have babies sleep on their backs.

A total of 52 children died last year from Sudden Infant Death Syndrome (SIDS), compared to 45 in 1996. It remains the largest single killer in the first year of life.

Details were announced by the Scottish Cot Death Trust alongside new research it is funding to investigate possible links with apnoea in adults - who stop breathing when asleep.

Trust chairman Dr Angus Gibson said: ''The increase is a very worrying development. Because we do not understand the mechanism of cot death, it is impossible to say why the rates are fluctuating in this way.''

Although the increase may be a statistical blip, there is other evidence to suggest a more widespread problem. England's figures for last year are expected to show a significant rise, but Ireland provides the most startling picture. Last year, its cot death rate doubled, accounting for 69 fatalities.

Dr Gibson said: ''We are not trying to raise an alarm story or mass hysteria campaign, but the facts do stand out. We must continue our research to identify what is actually happening to these babies.''

The potential similarities with apnoea are being investigated by Professor Neil Douglas of the respiratory medicine unit at Edinburgh Royal Infirmary. Its sleep laboratory already has considerable expertise working with adults.

Professor Douglas said an earlier study had shown far higher incidence of cot deaths in families with a sleep apnoea problem. This condition, also characterised by loud snoring, affects between 2 to 4% of middle-aged adults.

He said there could be an association between the irregular breathing patterns of adults and why young babies suddenly stop breathing.

''Our previous work has shown some similarities between the two conditions. The research into asleep apnoea will not only clarify the importance of upper airway obstruction in SIDS, but could also allow us to identify high risk infants,'' Professor Douglas said.

''We are not saying this is the cause of cot death. I am sure there are many causes. What we are doing is putting forward an hypothesis which will be tested,'' he added. He said half of sleep apnoea patients seemed to have an abnormal facial shape and bone structure, where the jaws were set back, compressing airways. In most cases, it was difficult to tell this without X-rays to show detailed facial anatomy.

The #70,000 research project will compare 100 parents who had suffered a cot death with 100 control subjects.

Professor Douglas said that if positive links were established, it might be possible to adapt treatments used for adults with sleep apnoea - which include wearing face masks to control breathing - for at-risk babies.

Dr Gibson said the fall in numbers of deaths during the 1990s was probably largely due to getting babies to sleep on their backs rather than their stomachs. Smoking remained a large risk factor, and social class also carried a risk factor - cot death was three times higher in poorest sections of society compared to most affluent.