Huntington disease (HD) is caused by a pathological elongation of CAG repeats in the huntingtin protein gene and is characterized by atrophy and neuronal loss primarily in the striatum. Mitochondrial dysfunction and impaired Ca2+ homeostasis in HD have been suggested previously. Here, we elucidate the effects of Ca2+ on mitochondria from the wild type (STHdhQ7/Q7) and mutant (STHdhQ111/Q111) huntingtin-expressing cells of striatal origin. When treated with increasing Ca2+ concentrations, mitochondria from mutant huntingtin-expressing cells showed enhanced sensitivity to Ca2+, as they were more sensitive to Ca2+-induced decreases in state 3 respiration and DeltaPsim, than mitochondria from wild type cells. Further, mutant huntingtin-expressing cells had a reduced mitochondrial Ca2+ uptake capacity in comparison with wild type cells. Decreases in state 3 respiration were associated with increased mitochondrial membrane permeability. The DeltaPsim defect was attenuated in the presence of ADP and the decreases in Ca2+ uptake capacity were abolished in the presence of Permeability Transition Pore (PTP) inhibitors. These findings clearly indicate that mutant huntingtin-expressing cells have mitochondrial Ca2+ handling defects that result in respiratory deficits and that the increased sensitivity to Ca2+ induced mitochondrial permeabilization maybe a contributing mechanism to the mitochondrial dysfunction in HD.