Huntingtin is an essential protein that with mutant polyglutamine tracts initiates dominant striatal neurodegeneration in Huntington's disease (HD). To assess the consequences of mutant protein when huntingtin is limiting, we have studied three lines of compound heterozygous mice in which both copies of the HD gene homolog (Hdh) were altered, resulting in greatly reduced levels of huntingtin with a normal human polyglutamine length (Q20) and/or an expanded disease-associated segment (Q111): Hdh(neoQ20)/Hdh(neoQ20), Hdh(neoQ20)/Hdh(null) and Hdh(neoQ20)/Hdh(neoQ111). All surviving mice in each of the three lines were small from birth, and had variable movement abnormalities. Magnetic resonance micro-imaging and histological evaluation showed enlarged ventricles in approximately 50% of the Hdh(neoQ20)/Hdh(neoQ111) and Hdh(neoQ20)/Hdh(null) mice, revealing a developmental defect that does not worsen with age. Only Hdh(neoQ20)/Hdh(neoQ111) mice exhibited a rapidly progressive movement disorder that, in the absence of striatal pathology, begins with hind-limb clasping during tail suspension and tail stiffness during walking by 3-4 months of age, and then progresses to paralysis of the limbs and tail, hypokinesis and premature death, usually by 12 months of age. Thus, dramatically reduced huntingtin levels fail to support normal development in mice, resulting in reduced body size, movement abnormalities and a variable increase in ventricle volume. On this sensitized background, mutant huntingtin causes a rapid neurological disease, distinct from the HD-pathogenic process. These results raise the possibility that therapeutic elimination of huntingtin in HD patients could lead to unintended neurological, as well as developmental side-effects.