The follow-up of neurophysiological tests (brain-stem auditory evoked potentials; blink reflex; sensory, motor and visual evoked potentials) and CT was investigated in 21 patients with late-onset cerebellar ataxia (CA) or multiple system atrophy. The study included an initial investigation and a follow-up examination on average 25.3 months later (minimum 8, maximum 36). Patients were divided into four groups: (1) those with pure CA after a minimum course of 5 years; (2) those with pure CA with pathological neurophysiological findings at the last examination; (3) those who at the first examination clinically presented with pure CA, but at the last examination were seen to have developed a multisystem disorder; (4) those with multiple system atrophy (mostly olivopontocerebellar atrophy) presenting additional non-cerebellar signs of involvement. Conforming to a strict interpretation of pure CA, group 1 patients invariably exhibited normal neurophysiological findings at all examinations. All patients in group 4, except for 2 only at the first examination, showed pathological changes in at least one of the neurophysiological tests. The main conclusion of this paper is that individuals who according to clinical criteria were initially classified as having CA but finally developed a multisystem disorder already had pathological neurophysiological findings at the initial examination (group 3). The increasing frequency of pathology in the several neurophysiological tests together with the progression of the disease is obviously of prognostic significance. CT revealed cerebellar atrophy without apparent involvement of brain-stem structures in all patients with CA; the majority of patients with multiple system atrophy also had atrophy of the brain-stem, pointing to olivopontocerebellar atrophy.(ABSTRACT TRUNCATED AT 250 WORDS)