Nasopharyngeal oxygen (NPO) therapy may overcome some of the difficulties associated with nasal prongs and facemask oxygen delivery devices. In response to a lack of published studies of NPO therapy in adults, we conducted a prospective randomised crossover trial to compare the effectiveness of NPO, nasal prongs (NP) and facemasks (FM) when used in an adult population (n = 37) from the intensive care unit and general hospital wards. We measured oxygen saturation (SpO2) using pulse oximetry, oxygen flow (litres per minute), respiration rate (per minute) and comfort using a horizontal visual analogue scale. All three devices were effective in maintaining a SpO2 of > or = 95% (NP 97.0 +/- 1.9, NPO 97.7 +/- 1.7, FM 98.8 +/- 1.3%). NPO therapy consumed less oxygen than NP and FM therapy (NP 2.6 +/- 1.0, NPO 2.2 +/- 0.9, FM 6.1 +/- 0.4 l/min, P < 0.001). There was no significant difference in patients' respiratory rates (NP 19.9 +/- 3.2, NPO 19.9 +/- 3.0, FM 19.8 +/- 3.1 per minute, P = 0.491). In terms of comfort, patients rated NP higher than NPO and FM using a horizontal visual analogue scale (100 mm = most comfortable) (NP 65.5 +/- 14.3, NPO 62.8 +/- 19.4, FM 49.4 +/- 21.4 mm, P < 0.001). We conclude that for adult patients, nasal prongs and nasopharyngeal oxygen therapy consume less oxygen and provide greater comfort than facemasks while still maintaining SpO2 > or = 95%.