We developed a battery of tests, incorporating motor function, olfaction, and mood, that distinguishes between early mild, probable idiopathic Parkinson's disease (iPD), and normal control subjects. Here, we determined the PD Battery's ability to prospectively identify which patients with symptoms and signs suggestive of, but not yet diagnostic of, iPD who would subsequently be diagnosed as having iPD. Of 212 enrolled patients, 194 have been followed for at least 1 year. Among these 194, 59 were diagnosed with iPD, 40 as neurologically normal, and the remaining 106 with other neurologic conditions such as essential tremor. The test battery was 92% specific and 68% sensitive for iPD. The area under the Receiver Operating Characteristics curve was 0.88 for predicting which participants would subsequently be diagnosed as having iPD or as being neurologically normal.