Background: Differences between the sexes among the non-elderly with heart failure (HF) have been insufficiently evaluated. This study aims to investigate sex-related differences in early-onset HF. Methods: Patients aged 18 to 54 years who were registered from 2003 to 2014 in the Swedish Heart Failure Register were included. Each patient was matched with two controls from the Swedish Total Population Register. Data on comorbidities and outcomes were obtained through the National Patient Register and Cause of Death Register. Results: We identified 3752 patients and 7425 controls. Of the patients, 971 (25.9%) were women and 2781 (74.1%) were men with a mean (standard deviation) age of 44.9 (8.4) and 46.4 (7.3) years, respectively. Men had more hypertension and ischemic heart disease, whereas women had more congenital heart disease and obesity. During the median follow-up of 4.87 years, 26.5 and 24.7 per 1000 person-years male and female patients died, compared with 3.61 and 2.01 per 1000 person-years male and female controls, respectively. The adjusted hazard ratios for all-cause mortality, compared with controls, were 4.77 (3.78-6.01) in men and 7.84 (4.85-12.7) in women (p for sex difference = 0.11). When HF was diagnosed at 30, 35, 40, and 45 years, women and men lost up to 24.6 and 24.2, 24.4 and 20.9, 20.5 and 18.3, and 20.7 and 16.5 years of life, respectively. Conclusion: Long-term mortality was similar between the sexes. Women lost more years of life than men.