BACKGROUND Aspirin is routinely prescribed in high-risk pregnancies to prevent pre-eclampsia; however, there is a paucity of data in women with pre-existing diabetes. OBJECTIVE To assess the efficacy and safety of aspirin in women with pre-existing diabetes in preventing pre-eclampsia. METHODS A retrospective review of women with pre-existing diabetes who attended antenatal clinics in a tertiary referral hospital between 2013 and 2019 was conducted. Cases were those receiving aspirin prior to 16 weeks, with pre-eclampsia as the primary outcome. The relationship between early pregnancy glycaemic control and pre-eclampsia was also assessed. RESULTS Of the 164 women included in the study, 45 received aspirin. There were no differences in pre-eclampsia (odds ratio (OR) 0.9 (0.3-3.0), P = 0.924) or any other measure of placental insufficiency (OR 1.7 (0.7-4.3), P = 0.243) between the aspirin and control groups after adjusting for baseline differences. Aspirin therapy was associated with an increased risk of postpartum haemorrhage (PPH) (OR 3.1 (1.1-9.1), P = 0.041). The incidence of pre-eclampsia increased stepwise according to early pregnancy HbA1c subgroups of ≤6.0% (n = 47), 6.1-7.5% (n = 57) and > 7.5% (n = 39), with rates of 0, 12.3 and 20.5% (P = 0.007) respectively. CONCLUSIONS The aspirin group had a higher baseline risk of pre-eclampsia and placental insufficiency, therefore the absence of difference between the groups favoured the efficacy of aspirin. PPH was highlighted as a potential complication of therapy, and early pregnancy HbA1c as a novel risk stratification tool for pre-eclampsia in women with pre-existing diabetes.