The aim of this study was to investigate seasonal variation in haemoglobin A1c (HbA1c) using a retrospective population analysis linking HbA1c values to the time of year. The study cohort included 171 442 NHS patients from Scotland, UK. Patients were divided into three groups. The stable group were defined as having a HbA1c that varied less than the tertile either side of the mean HbA1c. The high summer was comprised of patients with a HbA1c that varied by the second and third tertile above the mean HbA1c in summer. A high winter group was defined similarly but HbA1c being higher during the winter months. There was a significant temporal trend in the mean HbA1c levels for all patients in the study with a maximal variation of 2.4 mmol/mol (P<0.01) between May and August. In high summer patients, there was a variation of 7.5 mmol/mol (P<0.01) between January and October. In high winter patients, there was a variation of 8.7 mmol/mol (P<0.01) between March and September. Overall 45.3% of all patients (n=77 720) were in the high winter HbA1c group, 31.3% (n=53 656) were in the high summer HbA1c group, and 23.4% (n=40 066) were in the stable HbA1c group. Associations with seasonal variation of HbA1c included social deprivation, male gender, T1DM, increasing BMI and increasing diastolic blood pressure (DBP). The most likely explanation for this is treatment adherence, however this could not be measured in this retrospective population analysis. More than half the patients have a clinically relevant annual variation in HbA1c, which could impact on clinical decision making. Both clinicians and researchers need to be aware of the influence of time of year on the HbA1c value, especially in the context of meeting treatment target goals for HbA1c.