Aims: Prospective memory has been long considered a fundamental cognitive ability for optimal medication taking, but the role of prospective memory errors (termed ‘slips’) in diabetes self-care is unclear. Our aim was to examine associations between prospective memory and medication taking in adults with Type 1 and Type 2 diabetes mellitus. Methods: Some 901 adults with Type 1 diabetes and 927 with Type 2 diabetes completed a cross-sectional survey focused on the psychological and behavioural aspects of living with diabetes. Respondents reported whether they had forgotten to take their diabetes medication over the previous 14 days. Results: Twenty-four per cent (n = 220) of adults with Type 1 diabetes and 23% (n = 211) with Type 2 diabetes reported that they had forgotten their medication at least once over the previous 14 days. This was associated with more prospective memory slips in adults with Type 1 diabetes [odds ratio (OR) 1.09, 95% confidence interval (CI) 1.05 to 1.13; P < 0.001] and Type 2 diabetes (OR 1.10, 95% CI 1.05 to 1.15; P < 0.001); and with younger age (both groups), insulin pump use (Type 1 diabetes), insulin treatment (Type 2 diabetes), less frequent blood glucose checks (Type 1 diabetes) and higher HbA 1c (Type 1 diabetes). Conclusions: These findings suggest that forgetting medication is relatively common among adults with Type 1 or Type 2 diabetes, and provide preliminary evidence for its relationship with self-reported prospective memory slips.