The Official View

Asthma UK, a UK Asthma charity, has this to say about the Butekyo technique :

Very little has been published in medical journals about the Buteyko technique...

Two [Australian] studies ... found both positive and negative effects of the technique.

Asthma UK funded research ... of the Buteyko breathing method ...

It showed that for some people with asthma the Butekyo breathing technique helped to reduce their symptoms and use of reliever inhaler, but it did not improve their underlying condition.

Buteyko breathing may help people adapt to their asthma and feel more in control of their treatment and may be worth trying for those ... willing to commit the time required.

More research is needed to identify if certain people with asthma benefit more than others.

A review for the Austrailian Government, Breathing exercises for asthma (Holloway E, Ram FSF., Cochrane Review) found that they hasn't been enough research as the combined total of people involved in all Buteyko trials is less than 400.

It takes commitment for people to learn and practise these techniques regularly...

[we] have not identified any adverse effects ... from using these techniques.

... the current results [from trials] are encouraging ...

... there was some ... improvement for people who did the breathing training compared to those who did not, in every trial.

For more information about this and asthma in general : Austrialian Gov's Informed Health Asthma page