Patients who fail to maintain adequate respiration by themselves may need long-term mechanical ventilation (LTMV) for shorter or longer consecutive time periods. Patients who need LTMV are heterogeneous with respect to age, diagnosis and the progression of their disease. Recent Norwegian data also suggests considerable regional differences in the indication for initiation of LTMV. The Norwegian Directorate of Health requested the Norwegian Knowledge Centre for Health Services to review the effects of LTMV. In response we have prepared and are publishing three consecutive reports to address this question. This is the second report in the series in which we review the effects of LTMV for patients with obesity hypoventilation syndrome (OHS) and for patients with chest wall diseases. Obesity hypoventilation syndrom (OHS): We did not identify studies of the effect of LTMV and patient survival. LTMV-treatment may be associated with a decrease in the number of hospital admittances and improvements in some parameters related to quality of life and sleep. However, the quality of the evidence is very low and it is not possible to draw clear conclusions about the effectiveness of LTMV. Chest wall disease (CWD): LTMV may be associated with increased survival compared to LTOT (Long Term Oxygen Treatment) alone, but the quality of the evidence is low and any conclusion is uncertain. LTMV may be associated with fewer hospital admissions and improvements in some parameters related to quality of life and sleep. However, the quality of the evidence is very low and it is not possible to draw any clear conclusions based on the available evidence.