In view of reports that low-dose bromocriptine (15 mg or less daily) therapy is effective in Parkinson's disease, we undertook a retrospective study of 79 patients with Parkinson's disease to evaluate factors that affect the optimal daily dose of bromocriptine in subjects whose intake of levodopa was decreased as bromocriptine was introduced. Doses of bromocriptine 15 mg or less daily were seldom therapeutic; doses of over 15 mg, up to 30 mg, were appropriate for patients with mild, early disease. Those with more severe symptoms of longer duration usually required larger doses. The optimal daily requirement of bromocriptine was correlated to the severity and duration of the illness, but not to the age of the patient.