Objective To explore whether living alone and loneliness 1) are associated with poor patient-reported outcomes at hospital discharge and 2) predict cardiac events and mortality 1 year after hospital discharge in women and men with ischaemic heart disease, arrhythmia, heart failure or heart valve disease. Methods A national cross-sectional survey including patients with known cardiac disease at hospital discharge combined with national register data at baseline and 1-year follow-up. Loneliness was evaluated using one self-reported question, and information on cohabitation was available from national registers. Patient-reported outcomes were Short Form-12, Hospital Anxiety and Depression Scale and HeartQoL. Clinical outcomes were 1-year cardiac events (myocardial infarction, stroke, cardiac arrest, ventricular tachycardia/fibrillation) and all-cause mortality from national registers. Results A total of 13 443 patients (53%) with ischaemic heart disease, arrhythmia, heart failure or heart valve disease completed the survey. Of these, 70% were male, and mean age was 66.1 among women and 64.9 among men. Across cardiac diagnoses, loneliness was associated with significantly poorer patient-reported outcomes in men and women. Loneliness predicted all-cause mortality among women and men (HR 2.92 (95% CI 1.55 to 5.49) and HR 2.14 (95% CI 1.43 to 3.22), respectively). Living alone predicted cardiac events in men only (HR 1.39 (95% CI 1.05 to 1.85)). Conclusions A strong association between loneliness and poor patient-reported outcomes and 1-year mortality was found in both men and women across cardiac diagnoses. The results suggest that loneliness should be a priority for public health initiatives, and should also be included in clinical risk assessment in cardiac patients.