In experimental studies, the old mucolytic agent N-acetylcysteine (NAC) has had beneficial effects in disorders supposedly linked to oxidative stress. Numerous, mainly small clinical trials with variable doses have yielded inconsistent results in a wide variety of diseases. NAC added to the conventional therapy of human immunodeficiency virus infection might be of benefit; in respect of chronic obstructive pulmonary disease, systematic reviews and meta-analyses suggested that prolonged treatment with NAC is efficacious, but a recent multicentre study has questioned this. In a large intervention trial on cancer recurrence, NAC was ineffective. NAC infusions have been widely used in acute hepatic failure but convincing evidence of its benefits is lacking. A preliminary study reported that NAC is effective in preventing radiocontrast-induced nephropathy but thereafter highly mixed results have been published, and even meta-analyses disagree on its efficacy. In intensive care NAC has mostly been a disappointment but recently it has 'given promises' in surgery with cardiopulmonary bypass. NAC therapy is routine only in paracetamol intoxication.