Huntington's disease (HD) is associated with expansion of polyglutamine tract in a protein named huntingtin (htt) that is expressed in virtually all body tissues. Thus mutated htt (HD-htt) might affect all organs, although clinical manifestations of HD are associated with selective loss of corticostriatal neurons of the brain. In this work we studied how HD-htt affects mitochondria in human peripheral blood cells. We compared various functions of mitochondria isolated from cultured lymphoblastoid cells derived from three HD patients with juvenile onset of the disease (HD-LBM) and three age-matched control (C-LBM) individuals. Respiratory parameters in different metabolic states, with succinate and glutamate plus malate were the same for all control and HD cell lines. State 4 membrane potential in HD-LBM was slightly lower than in C-LBM. The calcium retention capacity (CRC) of mitochondria was estimated using simultaneously several methods to register permeability transition (PT). We found that LBM do not undergo swelling upon Ca2+-induced PT, and do not increase CRC in the presence of ADP + oligomycin. Although each cell line had different CRC values, qualitatively PT was different in C-LBM and HD-LBM. With C-LBM cyclosporin A (CsA) increased CRC significantly, while with HD-LBM CsA was ineffective. In C-LBM depolarization of mitochondria and a large pore opening (PT) always occurred simultaneously. In HD-LBM depolarization occurred at 20-50% lower Ca2+ loads than PT. We suggest that HD-htt promotes low H+ conductance of the mitochondria by interacting with proteins at the contacts sites without directly promoting PT or hampering mitochondrial oxidative phosphorylation.