Different varieties of pupillary abnormalities have been described in Parkinson's disease but it is not clear if this is due to the disease itself or to the pharmacological treatment. In this study, pupil reactivity to various stimuli (dark/light adaptation, light reflex, near vision reaction and electrical sural stimulation) was investigated in 23 de novo parkinsonian patients (not on drugs) to assess pupillary abnormalities and investigate their pathophysiological mechanisms. Parkinsonian patients exhibited larger pupil diameter after light adaptation, as well as a reduced amplitude of contraction and a prolonged contraction time at light reflex. The data obtained confirmed the existence in Parkinson's disease of a pupillary imbalance mainly involving the parasympathetic system.