OBJECTIVE: To detect the effectiveness of incremental speed-dependent treadmill training on postural instability, dynamic balance and fear of falling in patients with idiopathic Parkinson's disease. DESIGN: Randomized controlled trial. SETTING: Ankara Education and Research Hospital, 2nd PM&R Clinic, Cardiopulmonary Rehabilitation Unit. SUBJECTS: Fifty-four patients with idiopathic Parkinson's disease in stage 2 or 3 of the Hoehn Yahr staging entered, and 31 patients (21 training, 10 control) had outcome data. INTERVENTIONS: Postural instability of patients with Parkinson's disease was assessed using the motor component of the Unified Parkinson's Disease Rating Scale (UPDRS), Berg Balance Test, Dynamic Gait Index and Falls Efficacy Scale. Twenty-one patients with Parkinson's disease participated in an eight-week exercise programme using incremental speed-dependent treadmill training. Before and after the training programme, balance, gait, fear of falling and walking distance and speed on treadmill were assessed in both Parkinson's disease groups. MAIN MEASURES: Walking distance and speed on treadmill, UPDRS, Berg Balance Test, Dynamic Gait Index and Falls Efficacy Scale. RESULTS: Initial total walking distance of the training group on treadmill was 266.45 +/- 82.14 m and this was progressively increased to 726.36 +/- 93.1 m after 16 training session (P < 0.001). Tolerated maximum speed of the training group on treadmill at baseline was 1.9 +/- 0.75 km/h and improved to 2.61 +/- 0.77 km/h (P < 0.001). Berg Balance Test, Dynamic Gait Index and Falls Efficacy Scale scores of the training group were improved significantly after the training programme (P < 0.01). There was no significant improvement in any of the outcome measurements in the control group (P > 0.05). CONCLUSIONS: Specific exercise programmes using incremental speed-dependent treadmill training may improve mobility, reduce postural instability and fear of falling in patients with Parkinson's disease.