We studied 100 consecutive patients with Parkinson's disease (PD) who were not receiving levodopa and followed them until symptoms advanced and levodopa was given. Eighty-three patients eventually received levodopa; 50% started within 36 months after a mean duration of symptoms of 48 months. Patients starting on levodopa showed significant progression of their disease compared with their baseline examination. These data have direct applicability to the design and implementation of future protocols aimed at preventing disease progression of PD.