AIM/HYPOTHESIS: The aim of the study was to investigate mitochondrial function, fibre type distribution and substrate oxidation in arm and leg muscle during exercise in patients with type 2 diabetes and in obese and lean controls. METHODS: Indirect calorimetry was used to calculate fat and carbohydrate oxidation during both progressive arm-cranking and leg-cycling exercises. Muscle biopsies from arm and leg were obtained. Fibre type, as well as O(2) flux capacity of saponin-permeabilised muscle fibres were measured, the latter by high resolution respirometry, in patients with type 2 diabetes, age- and BMI-matched obese controls, and age-matched lean controls. RESULTS: Fat oxidation was similar in the groups during either arm or leg exercise. During leg exercise at higher intensities, but not during arm exercise, carbohydrate oxidation was lower in patients with type 2 diabetes compared with the other groups. In patients with type 2 diabetes, ADP-stimulated state 3 respiration per mg muscle with parallel electron input from complex I+II was lower in m. vastus lateralis compared with obese and lean controls, whereas no differences between groups were present in m. deltoideus. A higher percentage of type IIX fibres was seen in m. vastus lateralis in patients with type 2 diabetes compared with obese and lean controls, whereas no difference was found in the deltoid muscle. CONCLUSIONS/INTERPRETATION: This study demonstrates similar O(2) flux capacity, fibre type distribution and carbohydrate oxidation in arm muscle in the groups despite the presence of attenuated values in leg muscle in patients with type 2 diabetes compared with obese and lean controls.