Acoustic analysis of vocal instability during the production of isolated vowels, including computation of mean fundamental frequency (F0), period-to-period variability (jitter), pitch fluctuations, and between-trial variation of F0, was performed in 20 patients with cerebellar cortical dysfunction. Eleven subjects suffered from purely cerebellar atrophy (CA). In the remaining 9 patients olivopontocerebellar atrophy (OPCA) had been diagnosed. A subgroup of both the CA and OPCA subjects presented with enlarged pitch fluctuations and/or increased jitter values. It is conceivable that asymmetrically distributed motor deficits at the laryngeal level and altered gain settings of laryngeal and/or respiratory reflexes account for the observed phonatory instability. Moreover, 5 of the 20 cerebellar patients had a pitch level exceeding the upper limit of the normal range. Presumably, this deviation reflects increased vocal effort.