Breath acetone (BrAce) has been reported to be useful for monitoring the pathophysiology of patients with diabetes. However, devices that measure BrAce are expensive, complex and uncommon. The FM-001, originally designed to monitor a marker of weight loss in healthy people, is a device for measuring BrAce. The FM-001 is a loading semiconducting gas sensor that is a simple and reusable device. The aim of this study was to evaluate the correlation between blood total ketone bodies (TKB) and BrAce measured with the FM-001 in patients with diabetes. Furthermore, through evaluation of that correlation, we sought to detect patients at high risk of developing diabetic ketoacidosis (DKA). Thirty-five participants (age 52 [40–57], T2DM 32, T1DM 3) were enrolled. Scatter plots and linear regression lines relating BrAce to TKB and the correlation coefficients were calculated. Receiver-operating characteristic analysis was performed to determine the cut-off for predicting patients prone to DKA. The results showed that BrAce strongly correlates with TKB (R = 0.828), and the correlation was stronger in patients whose serum C-peptide was not low. The optimal BrAce cut-off for predicting risk of developing DKA was 3400 ppb (AUC 0.924, sensitivity 73.3%, specificity 100%), which corresponds to a TKB ⩾ 1000 µmol l−1. BrAce also weakly correlated with free fatty acid. Thus, BrAce levels measured with the FM-001 strongly correlate with TKB, even in patients with diabetes. This suggests the FM-001 is a simple and potentially useful method for detecting diabetic ketosis. (UMIN-ID: UMIN000038086)