The mechanisms by which neurons die in Huntington's disease (HD) are uncertain, however, mitochondrial dysfunction and apoptosis have been implicated. Because peripheral abnormalities may reflect similar consequences of mutant huntingtin in the brain, we evaluated markers of apoptotic cell death and mitochondrial function in peripheral blood cells of 10 HD patients and 16 age- and gender-matched controls. We found increased Bax expression in B and T lymphocytes, and monocytes from HD patients, but no alterations in Bcl-2 expression levels. B lymphocytes also showed decreased mitochondrial membrane potential. However, HD peripheral blood cells showed no differences in reactive oxygen species (ROS) levels when compared to controls. Our results suggest that peripheral blood cells, in particularly B lymphocytes may reflect changes observed in HD brain.