Sleep-disordered breathing (SDB) is common in patients with heart failure (HF) and leads to disturbed sleep. The objective of this study was to determine the persistent effects of long-term nocturnal oxygen treatment in patients with severe HF regarding (1) objective outcomes, such as sleep, SDB, cardiac function, and functional capacity; (2) subjective outcomes, such as self-assessed sleep difficulties, daytime sleepiness, and health-related quality of life (HRQOL); and (3) the relationship between objective and subjective outcomes. In this open nonrandomized experimental study, 22 patients, median age 71 years, with severe HF were studied before and after 3 months of receiving nocturnal oxygen. The measures used were overnight polysomnography, echocardiography, 6-minute walk test, self-assessed sleep difficulties (Uppsala Sleep Inventory-HF), daytime sleepiness (Epworth Sleepiness Scale), and HRQOL (36-Item Short Form Health Survey and Minnesota Living with Heart Failure Questionnaire). SDB, with a 90% dominance of central sleep apnea, occurred in 41% of the patients with severe HF before intervention. After intervention, functional capacity improved for both the whole group of patients with HF (P < .01) and HF patients with SDB (P < .05). No improvements regarding cardiac function, objective sleep, subjective sleep, or SDB were seen, except for a decrease of > or = 4% desaturations (P < .05). HRQOL did not differ significantly between HF patients with and without SDB before or after intervention with nocturnal oxygen. Long-term nocturnal oxygen treatment improved functional capacity in patients with severe HF, with or without SDB. No improvements were seen regarding sleep, daytime sleepiness, SDB, cardiac function, or HRQOL.