Objective For early differential diagnosis of slowly progressive type 1 diabetes mellitus (SPIDDM) from type 2 diabetes mellitus, we investigated clinical history and characteristics of patients with SPIDDM with or without obesity. Research design and methods Subjects were 39 patients (20 men and 19 women) diagnosed with SPIDDM based on positive antibodies against glutamic acid decarboxylase (GAD) and/or islet-associated antigen 2 (IA-2), with more than 3 months from diabetes mellitus (DM) diagnosis to initiation of insulin therapy, and who were inpatients at our hospital between January 2004 and July 2010. Body mass index (BMI) before DM diagnosis, clinical characteristics at the time of DM diagnosis and SPIDDM diagnosis, and C-peptide levels at the time of DM diagnosis were extracted from medical records. Results The proportion of the patients with maximum BMI C 25 kg/m 2 was 67 % before DM diagnosis, 53 % at the time of DM diagnosis, and 25.6 % at the time of SPIDDM diagnosis, whereas those with BMI \ 22 kg/m 2 were 10.3, 25.0, and 59.0 %, respectively. Compared with patients with BMI\22 kg/m 2 at the time of DM diagnosis, those with BMI C 25 kg/m 2 had a significantly longer period from DM diagnosis to initiation of insulin therapy (p = 0.026); all of these patients had a C-peptide index (CPI) B 1.0 at the time of SPIDDM diagnosis.Conclusion It is proposed that islet-specific autoantibodies should be measured in cases of obese patients with CPI B 1.0 for earlier diagnosis of SPIDDM.