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 < 0.001) in patients treated for HF, compared with patients treated for DM, when adjusted for the subject's age, sex, and cognitive function. CONCLUSION: In this study it was shown that patients' understanding of their diagnosis was highly dependent on diagnosis, independently of age, sex, or cognitive function. The results suggest that there is room for improvement of care in primary healthcare, to increase HF patients' understanding of their diagnosis.