Background: Adult-onset Still's disease (AOSD) is a systemic inflammatory disorder of unknown aetiology usually affecting young adults. Interestingly, recent evidence from the juvenile counterpart of AOSD suggested the emergent high fatality rate of lung disease (LD) in these patients. In this work, we aimed to characterise LD in AOSD, to identify associated clinical features and predictive factors, and to describe long-term outcomes of the disease comparing patients with LD and those without. Methods: A retrospective assessment of prospectively followed patients, from January 2001 to December 2019, was provided to describe the rate of LD in AOSD, associated clinical features and predictive factors, and long-term outcomes. Patients with AOSD, who were included in Gruppo Italiano di Ricerca in Reumatologia Clinica e Sperimentale (GIRRCS) cohort, were assessed. Results: Out of 147 patients included in GIRRCS cohort, 18 (12.25%) patients were reported to be affected by LD, at the time of diagnosis of AOSD, who were characterised by older age, a higher prevalence of myalgia, of lymph node involvement, of pleuritis, and abdominal pain. Furthermore, patients with LD showed higher values of systemic score and ferritin. Among those clinical variables, older age and systemic score were also independently predictors of LD. Chest CT scans were also obtained, and the most common finding was the peripheral consolidations in 8 (44.4%) patients. Finally, a higher mortality rate, of 38.9%, was registered in patients with LD than others, since it was associated with a significant decreased survival rate. Conclusions: The presence of LD could suggest an emergent cause of mortality in AOSD, as observed in juvenile counterpart recognising a further marker of severity and poor prognosis to be careful evaluated. Patients with LD were also characterised by some clinical features, higher values of systemic score and ferritin than the others, identifying a subset of patients mostly burdened by systemic signs and symptoms. Although specific designed future studies are needed to fully elucidate the significance of LD in AOSD, a more accurate evaluation and management of this feature could improve the long-term outcomes of these patients.