Objectives: Patients with Parkinson's disease (PD) begin to lose weight several years before diagnosis, which suggests weight variation is associated with some factor(s) that precede the onset of motor symptoms. This study aimed to investigate the association of autonomic nervous system with body weight in patients with PD. Materials and methods: The subjects were 90 patients with early de novo PD. We examined the associations of body mass index (BMI) with sympathetic nervous - activity reflected in orthostatic intolerance or cardiac uptake of I-123-metaiodobenzylguanidine and parasympathetic nervous activity reflected in constipation or heart rate variability (HRV). Results: Twelve patients (13.3%) were overweight (BMI>25 kg/m(2)), 62 patients (68.9%) were normal-weight (18.5 <= BMI<25 kg/m(2)), and 16 patients (17.8%) were underweight (BMI<18.5 kg/m(2)). Underweight patients had greater disease severity and decrease in blood pressure on head-up tilt-table testing, higher cardiac washout ratio of I-123-metaiodobenzylguanidine, and lower HRV and complained of constipation more often than those with normal-weight or overweight patients. On multiple regression analyses, the correlation of these variables with BMI maintained statistical significance after adjustment for age, sex, symptom duration, and motor subtype. Conclusions: Dysautonomia and disease severity are closely related to body weight independently of age, sex, symptom duration, and motor subtype. Dysautonomia may play a partial role on weight variation in the early stage of PD.