The effects of the acute administration of clozapine on parkinsonian mixed tremor (i.e., resting and postural tremors) were evaluated to establish clozapine's predictive value for long-term response and to determine if there is a difference in the pharmacologic responses of the two tremors. We also investigated the correlation between reduction of tremor and induction of sedation after acute and chronic administration of clozapine. Clozapine (12.5 mg) or placebo were administered po in a double-blind manner to 17 PD patients with mixed L-dopa-resistant tremors. Two patients did not reach 50% improvement and were considered nonresponders. The remaining 15 patients reported moderate to marked reduction of tremor. Responsive patients in the acute test moved on to a long-term, open clozapine add-on study receiving an average daily dose +/- SD of 45 +/- 9.6 mg for a period of 15.5 +/- 8.3 months. A significant reduction of both resting (p < 0.05) and postural (p < 0.05) tremors was observed under clozapine from the first week of treatment through the entire period of the study. There was no statistically significantly difference between the degree of improvement for resting and postural tremors after either single or chronic clozapine administration. Sedation was the only side effect reported after clozapine; however, the time courses of sedation and tremor reduction did not coincide in the acute or in the chronic experimental paradigm, where it decreased considerably in a few weeks in all patients. During long-term clozapine treatment, neither systemic side effects nor worsening of motor disability scores were noted. Thus we wish to propose an acute test or a therapeutic attempt, or both, with clozapine before defining a case of mixed parkinsonian tremor as resistant tremor and therefore resorting to a neurosurgical approach.