Background: Serum 1,5-anhydroglucitol (1,5-AG) levels are a measure that provides information on daily glycemic variations. We evaluated whether 1,5-AG could be a possible marker of acute ischemic stroke (AIS) or transient ischemic attacks (TIA) in patients with diabetes mellitus (DM). Methods: We retrospectively reviewed electronic medical records of 5,294 AIS/TIA patients. Of the 5,294, 1,898 had diabetes and in 1,246, serum 1,5-AG levels were measured (group S). Group S was divided into 2 subgroups: hemoglobin A1c (HbA1c) <7% (S-low) and >7% (S-high). As controls, 394 outpatients with diabetes (group C) without AIS/TIA were likewise divided into subgroups, C-low and C-high according to HbA1c level. In each HbA1c subgroup, the association between serum 1,5-AG (≥14 vs. <14 µg/mL) and stroke was examined using multivariable logistic regression (MLR) with stepwise variable selection. In model 1, the OR and 95% CI was examined adjusted for age and gender. Known risk factors for stroke; hypertension, dyslipidemia, alcohol consumption, smoking, and estimated glomerular filtration rate were included in model 2. Results: Overall, serum 1,5-AG levels were lower in group S than in group C. Serum 1,5-AG levels were low in subgroups S-high and C-high, showing no differences in mean values. However, mean serum 1,5-AG levels in S-low was statistically lower than that in C-low. MLR analysis showed that the OR for low (<14 µg/mL) 1,5-AG for stroke was statistically significant only in well-controlled diabetes (OR [95% CI] 2.19 [1.54–3.10]) in model 1 and (2.26 [1.56–3.28]) model 2. Conclusions: Low serum 1,5-AG levels could be a possible marker for AIS/TIA risk in patients with well-controlled DM.