Objective To explore the long-term therapeutic effects of co-transplantation of Sprague-Dawley (SD) rats islets and bone marrow mesenchymal stem cells (BMSCs) to treat type 1 diabetes mellitus (T1DM). Methods BMSCs were isolated from 1 week of age of 4 healthy male SD rats while islets were isolated from 6-8 weeks of age of 3 healthy male SD rats BMSCs and islets were both cultured with 1640 nutrient medium in either separately or together in vitro. The cell morphology, levels of insulin and C-peptide in each group were examined at the 1st, 3rd, and 7th day after culture. T1DM model (male SD rats, body weight 180-200 g) was established by administration of 1% streptozotocin (STZ) citrate buffer (60 mg/kg) through intraperitoneal injection. T1DM model rats were randomly divided into 4 groups: model group, BMSCs transplantation group, islets transplantation group, and co-transplantation group (15 rats each group) for the cell transplantation test in vivo. The fasting glucose, body weight, insulin, C-peptide, total bilirubin (TBIL), aspartate transaminase (AST) and alanine aminotransferase (ALT) of each group were examined at the 1st, 3rd, 7th, 14th, and 30th day after transplantation. Data between multigroups were analyzed by ANOVA, and data between two groups were analyzed by t test. Results Compared to islets transplantation group, blood glucose in co-transplantation group's was lower (t=11.44-28.02, P<0.05), and insulin (t=10.03-6.86, P<0.05) and C-peptide (t=1.47-2.59, P<0.05). However, the TBIL (t=7.67-6.39, P<0.05), AST (t=12.00-8.89, P<0.05), and ALT (t=8.08-10.40, P<0.05) in co-transplantation group were lower than those in islets transplantation group. The expression intensity of insulin was stronger in co-transplantation group than islets transplantation group. The differences of above indexes between model group and BMSCs group had no statistical significance (all P>0.05). Conclusion Compared with islets transplantation alone, islets and BMSCs co-transplantation have better therapeutic effects on T1DM. [ABSTRACT FROM AUTHOR]