A retrospective study at the University of the Basque Country analyzed 40 patients with post-COVID syndrome (32 women, 8 men) who underwent 10 sessions of large autohemotherapy with ozone. Beck Anxiety Inventory and Beck Depression Inventory scores were significantly reduced after treatment (p < 0.05). Fatigue measured by the FACIT Fatigue Scale decreased statistically significantly after 10 sessions (p < 0.05). All Short Form-36 quality of life subscores improved after ozone treatment (p < 0.05). The study also used the Pittsburgh Sleep Quality Index, but specific changes in sleep quality were not specified. The authors conclude that patients with post-COVID syndrome may benefit from major blood ozonation as adjunctive therapy after COVID-19.