OBJECTIVE: The objective was to study primary healthcare patients' understanding of their diagnosis of heart failure (HF), using patients treated for diabetes mellitus (DM) as a comparative group. DESIGN: A cross-sectional community based study. SETTING: Karlskrona community situated on the Swedish south-east coast with 60,600 inhabitants. SUBJECTS: A total of 1402 subjects, aged 60-96 years in 10 age cohorts, selected randomly from the national population registry participating in the Swedish National study on Ageing and Care - Blekinge. MAIN OUTCOME MEASURES: Understanding of diagnosis of HF or DM in primary healthcare. Prevalence of cognitive impairment. RESULTS: In all, 39.8% of patients with a diagnosis of HF treated in primary healthcare and 97.1% of patients with DM had an understanding of their respective diagnosis. Cognitive impairment was significantly more prevalent in the groups of patients treated for HF (OR 1.9, 95% CI 1.2 to 3.0) and DM (OR 1.8, 95% CI 1.1 to 3.1), when compared with those not treated for either HF or DM. The odds ratio for understanding of diagnosis was 0.013 (95% CI 0.003 to 0.052, p