Parkinson's disease evolves slowly, and there is current interest in exploring the earliest stages of the disorder, because of new approaches to studying pathogenesis and developing potential neuroprotective treatment. Recognizing early Parkinson's disease is not easy. The certainty of diagnosis increases as the disease advances. To address the problem of identifying Parkinson's disease in its initial phases of clinical expression, we propose the following designated levels of confidence for the diagnosis: (1) clinically possible, (2) clinically probable, and (3) clinically definite. Laboratory support for the diagnosis may be applied to each category. Criteria are provided as a framework underpinning this classification.