One hundred patients with Parkinson's disease (PD) and five patients with progressive supranuclear palsy were questioned about the frequency, circumstances, and consequences of falling. Parkinsonian symptoms were scored using the unified rating scale. Thirty-eight percent of parkinsonian patients fell, and 13% fell more than once a week. Broken bones (13%), hospitalization (18%), confinement to wheelchair (3%), and fear of walking occurred. Postural hypotension was uncommon and did not correlate to falling. Sensory loss, dementia, heart disease, and the use of antihypertensive medications were not related to falling. Falling did correlate with postural instability, bradykinesia, and rigidity but not with tremor. Falling was also related to age and duration of disease. The frequency of falling was correlated only to the severity of one parkinsonian symptom, postural instability. Progressive supranuclear palsy patients fell often and had marked postural instability. Factor analysis of parkinsonian characteristics yielded three groups, with tremor being an independent symptom. Frequent fallers and postural instability were not changed by dopaminergic therapy. Some fallers with gait difficulties and bradykinesia were improved with levodopa. Physical therapy was also of benefit to some patients. It is concluded that falling is a common problem in PD and may cause serious disability. Falling may be related to all the major motor signs except for tremor. Frequent falling is caused by postural instability, which is not reversible with dopaminergic therapy.