Mean body mass index (BMI) in 100 cases of idiopathic Parkinson's disease (PD) was found to be 9% reduced in comparison to that in patients with either essential tremor or no neurologic disease. A similar reduction in BMI was also discovered among the 24 cases of PD in whom retrospective BMI data were available from their presymptomatic years. These results suggest that alterations in nutrient intake or metabolism could reflect early changes in the central autonomic network preceding the emergence of classical extrapyramidal manifestations of PD.