Postural reflexes in response to sudden toe-up tilts of a supporting forceplate platform were studied in free-standing patients with stage III (N = 5) and stage IV (N = 5) Parkinson's disease and compared to 5 age- and sex-matched normals. Latencies of the short (SL), medium (ML) and long latency (LL) responses were normal in the patients. The normalized mean amplitudes of the ML responses were significantly increased only in the stage IV Parkinson patients (P less than 0.0005). The distal-proximal activation sequence of LL responses observed in all 5 controls was reversed in 1 of the stage III and 4 of the stage IV Parkinson patients. Both the enlargement of the ML response (P less than 0.01) and the inverted activation sequence of LL responses (P less than 0.01) were significantly correlated with severity of the disease. The data establish an association between abnormal modulation of postural reflexes in the lower extremity and clinically rated balance impairment in Parkinson's disease.