Background and aims: the relationship between hypoglycaemia and day-to-day variability of glycaemic control has not yet been well established. This post hoc analysis aimed to investigate the correlation between the day-to-day variability of fasting self- measured plasma glucose (SMPG) and hypoglycaemia in patients with type 1 (T1D) and type 2 diabetes (T2D). Materials and methods: a post hoc analysis was performed correlating day-to-day variability of fasting SMPG with hypoglycaemia in two double-blind, treat-to-target, crossover trials that compared insulin degludec once daily (OD) with insulin glargine U100 OD in adults with T1D (SWITCH 1, n=501) or insulin-experienced adults with T2D (SWITCH 2, n=721). Available pre-breakfast SMPG measurements were used to determine a weekly variance for each patient, using the log SMPG values to allow for relative comparisons. For each patient and treatment, the geometric mean of the weekly variance was calculated and these values were categorised into low, medium and high tertiles, as a measure for day-to-day variability. The effect of having low or high variability compared with medium variability was analysed in relation to overall symptomatic (severe or blood glucose).