Abstract： Objective To describe the clinical characteristics and classification diagnosis of newly diagnosed diabetes onset with ketosis or ketoacidosis in adult patients.Methods Medical records of newly diagnosed diabetes onset with ketosis or ketoacidosis in the Third Affiliated Hospital of Sun Yat-sen University between January 2011 and August 2016 were reviewed.Patients aged 18 years or older were included,while other diseases that may cause urinary ketoacidosis and special types of diabetes were excluded.Patients were classified as type 1 diabetes mellitus (T1DM) or type 2 diabetes mellitus (T2DM) or diabetes mellitus untyped based on discharged diagnosis,and groups were compared for differences in clinical profiles.Then the patient's medication,final diagnosis and outcome within 2 years of discharge were tracked through the inpatient and the outpatient medical record systems.Receiver operating characteristics (ROC) curves were analyzed to check the ability of clinical indicators such as onset age,body mass index (BMI) and C-peptide to discriminate T1DM from T2DM,and to find the best diagnostic cut-off points.Results A total of 123 patients (88 males) were enrolled [with a mean age of (41.1±13.6) years old],with 37 patients (30.1％) diagnosed as T1DM,60 patients (48.8％) diagnosed as T2DM and 26 patients (21.1％)diagnosed as Untyped.There was a statistically significant difference in onset age,BMI,blood pressure,blood gas pH and bicarbonate,blood lipids,fasting,0.5 h and 2 h C-peptide level,any diabetic antibody and anti-glutamic acid decarboxylase antibody (GADA) positive rate,combined fatty liver ratio and family history among the three groups (all P＜0.05).ROC curve analysis was performed on patients diagnosed with T1DM (n=36) and T2DM (n=87) after 2 years follow-up,and the area under the curve (AUC) of onset age,BMI,fasting C-peptide,0.5 h and 2 h C-peptide was 0.735,0.813,0.855,0.898,and 0.882,respectively.Conclusion The ROC curve analysis indicates that C-peptide,onset age and BMI can provide effective diagnostic value,and the diagnostic value of C peptide is better than BMI and onset age.