Chronic constipation is the most frequent symptom of autonomic system involvement in Parkinson's disease (PD). Quite often the symptom is severe and impairs patients' quality of life. The objective of this study is to determine the efficacy and safety of an isosmotic macrogol solution for the treatment of constipation in PD patients, in a double-blind, placebo-controlled study. A total of 57 PD patients with constipation were randomly assigned to receive an isosmotic macrogol electrolyte solution (MC-ES; 29 patients) or placebo (28 patients) for 8 weeks. Treatment efficacy was defined as complete relief of the symptom or a marked improvement of two of the following indicators: stool frequency, straining, stool consistency, use of rectal laxatives as a rescue therapy. The responder rates were significantly higher in the MC-ES group both at the first (4 weeks; P < 0.0003) and at the final evaluation (8 weeks; P < 0.0012). The frequency of bowel movements (P < 0.002) and stool consistency (P < 0.006) were significantly changed in the MC-ES group compared to the placebo group. At the final evaluation, a rectal laxative was used by 2 (12.5%) patients on placebo, whereas no use was recorded in the MC-ES group. Responder rate for straining showed a favorable trend in patients treated with macrogol versus placebo. Unified Parkinson's Disease Rating Scale Part III and Parkinson's Disease Questionnaire (PDQ-39) did not show any significant modification in either group during the 8-week treatment period. The results of this placebo-controlled study show the efficacy of MC-ES in the treatment of constipation in PD. MC-ES was well-tolerated and did not affect the course of PD.