Long-term oxygen therapy (LTOT) in patients with chronic hypoxaemia is an expanding field. Traditional stationary oxygen, with and without portable compressed gas cylinders, and portable liquid oxygen equipment is available today. No official guidelines exist for the prescription of liquid oxygen vs traditional oxygen supply in LTOT, although it is generally though that patients receiving liquid oxygen are younger and less seriously affected. The authors tested this hypothesis by comparing register data from two national Scandinavian registers of patients on LTOT in the same time period; one including all the patients on traditional oxygen treatment alone, e.g. concentrators and compressed gas cylinders (n = 1039), the other including all the patients using portable liquid oxygen alone (n = 117). About 80% of the patients in both groups suffered from chrome obstructive pulmonary disease. Younger patients were found in the liquid oxygen group (P < 0.004), but with clinically slightly worse blood gas derangement (mostly statistically significant). They had higher frequency of previous or present oedema (P < 0.0001), and there were more smokers in this group (P < 0.0001). No significant differences were seen with respect to sex distribution or oxygen delivery systems. Higher oxygen dose and longer daily oxygen treatment (P < 0.0001) were prescribed for the liquid oxygen patients compared with the patients on traditional oxygen with statistically higher oxygen tension during the prescribed treatment. A slight further increase in PaCO2 was seen in both groups during oxygen treatment, of doubtful significance. Compared with patients on traditional oxygen, liquid oxygen thus appears to be prescribed for younger patients, independent of clinical status or blood gas levels.