Variable expression of Parkinson's disease: a base-line analysis of the DATATOP cohort. The Parkinson Study Group. The DATATOP database, which includes clinical information on 800 patients with early untreated Parkinson's disease (PD), is well suited to explore clinical heterogeneity in PD. Patients with early-onset PD (less than or equal to 40 years, N = 33) reached the same level of disability as the late-onset PD (greater than or equal to 70 years, N = 85) group at a significantly slower rate (2.9 vs. 1.7 years). Early-onset PD patients functioned cognitively better than late-onset PD patients. Bradykinesia, and postural instability and gait difficulty (PIGD), were more common at onset in patients with a rapid rate of disease progression ("malignant PD"; duration of symptoms less than 1 year and Hoehn/Yahr stage of 2.5, N = 11) as compared with those with a relatively slow rate of progression ("benign PD"; duration of symptoms greater than 4 years, N = 65). Comparisons of tremor-dominant PD (mean tremor score/mean PIGD score less than or equal to 1.5, N = 441) with the PIGD-dominant type (mean tremor score/mean PIGD score greater than or equal to 1.0, N = 233) provided support for the existence of clinical subtypes. The PIGD group reported significantly greater subjective intellectual, motor, and occupational impairment than the tremor group. Stage II patients had higher depression scores than stage I patients. Among the patients participating in the DATATOP, older age at onset with bradykinesia, or with the PIGD form of PD, is associated with more functional disability than when the symptoms are dominated by tremor or begin at a younger age. 