Objective: To identify the factors associated with prolonged length of hospital stay in patients admitted for acute heart failure. Methods: Multipurpose observational cohort study including patients from the EAHFE registry admitted for acute heart failure in 25 Spanish hospitals. Data were collected on demographic and clinical variables and on the day and place of admission. The primary outcome was length of hospital stay longer than the median. Results: We included 2,400 patients with a mean age of 79.5 (9.9) years; of these, 1,334 (55.6%) were women. Five hundred and ninety (24.6%) were admitted to the short stay unit (SSU), 606 (25.2%) to cardiology, and 1,204 (50.2%) to internal medicine or gerontology. The mean length of hospital stay was 7.0 (RIC 4-11) days. Fifty-eight (2.4%) patients died and 562 (23.9%) were readmitted within 30 days after discharge. The factors associated with prolonged length of hospital stay were chronic pulmonary disease; being a device carrier; having an unknown or uncommon triggering factor; the presence of renal insufficiency, hyponatremia and anaemia in the emergency department; not being admitted to an SSU or the lack of this facility in the hospital; and being admitted on Monday, Tuesday or Wednesday. The factors associated with length of hospital stay <= 7days were hypertension, having a hypertensive episode, or a lack of treatment adherence. The area under the curve of the mixed model adjusted to the center was 0.78 (95% CI: 0.76-0.80; p < 0.001). Conclusions: A series of factors is associated with prolonged length of hospital stay and should be taken into account in the management of acute heart failure.