Symptoms of nocturnal hypoventilation may negatively influence the quality of life (QoL) of ALS patients long before respiratory failure ensues. Non-invasive mechanical ventilation (NIV) is considered a treatment option for nocturnal hypoventilation. The primary objective of NIV is improving quality of life (QoL). It may also prolong life by several months. A systematic review of the literature was performed to analyse what is known of the effect of NIV on survival, QoL and other outcome measures. A computerized literature search was performed to identify controlled clinical trials and observational studies of treatment of ALS-associated nocturnal hypoventilation from 1985 until May 2005. Twelve studies fulfilled the inclusion criteria. Four studies were retrospective, seven prospective and in one study randomization was used. All studies reported beneficial effects of NIV on all outcome measures. In seven studies NIV was associated with prolonged survival in patients tolerant for NIV, and five studies reported an improved QoL. In conclusion, studies on the use of NIV in ALS differ in study design and endpoint definitions. All studies suggest a beneficial effect on QoL and other outcome measures (Evidence level Class II-III). Well-designed randomized controlled trials comparing the effect on QoL and survival have not been performed.