Objective Research demonstrates that patients have a poor understanding of glycosylated haemoglobin A1c (HbA1c) and that this impacts on effective diabetes self-management. This study attempted to replicate these findings in a UK outpatient sample of people with diabetes. Method 83 participants were recruited and asked to fill in a questionnaire assessing their understanding of HbA1c, diabetes self-care behaviours and diabetes-specific self-efficacy in relation to carrying out these self-care behaviours. Results Only 26.5% of the participants were classified as having a good understanding of HbA1c. Correlational and univariate analyses indicated that this level of understanding was related to demographic variables, HbA1c levels and certain aspects of self-care and self-efficacy. A series of multiple regressions found that understanding was a significant predictor of HbA1c levels. Conclusion The majority of participants seemed to have a poor understanding of HbA1c and this was related to aspects of their diabetes management, self-efficacy and HbA1c levels. Practical implications These findings provide support for the application of programmes and initiatives aimed at improving patients understanding of clinical disease markers.