The Swedish Society of Chest Medicine has started a national register of patients on home mechanical ventilation, to establish reliable national prevalence data and to accurately document patient and treatment characteristics to enable a scientific evaluation of this treatment. In this first collection of retrospective register data, covering patients on home mechanical ventilation at the register start on 1 January 1996, we found 541 patients, corresponding to 6.1/100000 inhabitants, using home mechanical ventilation. Non-invasive ventilation, night-time ventilation and volume controlled ventilation dominated. We found four diagnosis categories of approximately equal size, namely post-polio, chest wall deformities, neuromuscular diseases and 'other diseases'. The age distribution was bimodal, with one small peak in the 20-29 year group and a large peak in the 60-69 year group. A survey of Danish patients on home mechanical ventilation showed that they were considerably younger and that almost half of them suffered from neuromuscular diseases. Further work will be done to follow the situation in Sweden and in Denmark to elucidate the obvious differences in the selection of patients for home mechanical ventilation.