Patients who fail to maintain adequate respiration by themselves may need long-term mechanical ventilation (LTMV) for shorter or longer periods. Recent Norwegian data suggests considerable regional differences in the indication for initiation of LTMV. Norwegian Knowledge Centre for the Health Services has prepared three consecutive reports about LTMV. This is the third report in the series in which we review the effects of LTMV for patients with cystic fibrosis or COPD. The report overview two systematic reviews – one about the effect of LTMV for patients with cystic fibrosis and one about LTMV for patients with stable COPD: Current evidence does not suggest that LTMV is associated with improved survival or reduced need for hospitalisation in patient with stable COPD, but the quality of the evidence is too low to allow firm conclusions. LTMV probably have little or no effect on arterial blood gas values among patients with stable COPD. We need more evidence before concluding how LTMV affect sleep efficiency and quality of life in patients with stable COPD, and whether the effect of LTMV among patients with stable COPD is affected by the time selected for initiation of therapy. We need more evidence before concluding about the effectiveness of LTMV on survival, hospitalisation, sleep efficiency and quality of life in patients with cystic fibrosis.