Background: Huntington's disease (HD) is an autosomal dominant disease with neurologic manifestations. In transgenic mouse models of HD, weight loss is recognized as a feature associated with the disease onset. It is unclear whether a similar pattern occurs in humans. Methods: Data from the Huntington Study Group were used to evaluate whether HD is associated with lower body mass index (BMI) at the earliest stage of the disease. There were 361 case subjects in whom HD had been diagnosed with an independence scale rating of 100 (no special care needed), a total functional capacity score of ≥11, and HD duration of <4 years. For each case subject, five sex- and age-matched control subjects were selected from the National Heart, Lung, and Blood Institute Family Heart Study or the Framingham Offspring Study. Results: Among case subjects, neither disease duration, nor dystonia, nor chorea score was significantly associated with BMI. BMI was significantly lower among case than among control subjects. Among men, age-adjusted BMI (±SE) was 25.90 ± 0.34 kg/m2 for case subjects with HD and 27.68 ± 0.16 kg/m2 for control subjects. Among women, corresponding values were 24.34 ± 0.43 for case subjects with HD and 26.63 ± 0.21 kg/m2 for control subjects. Conclusions: At an early stage of the disease, subjects with Huntington's disease had lower body mass index than matched controls from the general population. The cause of weight loss is unknown but the parallel to observations in Huntington's disease transgenic mice suggests that it is a significant hallmark of Huntington's disease gene expression.