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Background: Maternally inherited diabetes and deafness (MIDD), which is seen in 0.5% to 2.8% of patients with type 2 diabetes mellitus, is related to a point mutation at position 3243 of mitochondrial (mt) DNA. Its clinical description is incomplete., Objective: To study the clinical presentation and complications of diabetes in patients with MIDD and to identify clinical characteristics that may help select diabetic patients for mtDNA mutation screening., Design: Multicenter prospective descriptive study., Setting: 16 French departments of internal medicine, diabetes and metabolic diseases, or both., Patients: 54 patients with type 2 diabetes mellitus and the mtDNA 3243 mutation., Measurements: Characteristics of diabetes, metabolic control (glycosylated hemoglobin level), complications of diabetes, and involvement of other organs., Results: On average, patients with MIDD were young at diabetes onset and presented with a normal or low body mass index. None were obese. Seventy-three percent of probands had a maternal family history of diabetes. Diabetes was non-insulin-dependent at onset in 87% of patients; however, 46% of patients had non-insulin-dependent disease at onset but progressed to insulin therapy after a mean duration of approximately 10 years. Neurosensory hearing loss was present in almost all patients. Eighty-six percent of patients who received an ophthalmologic examination had macular pattern dystrophy (a specific retinal lesion). Forty-three percent of patients had myopathy, 15% had cardiomyopathy, and 18% (9 of 51) had neuropsychiatric symptoms. Although the prevalence of diabetic retinopathy was 8% among patients who received an ophthalmologic examination, lower than expected after a mean 12-year duration of diabetes, prevalence of kidney disease was 28%. This suggests that a specific renal involvement was the result of mitochondrial disease., Conclusions: Maternally inherited diabetes and deafness has a specific clinical profile that may help identify diabetic patients for mtDNA testing.