Persons symptomatic and at risk for Huntington's disease (HD) from a large extended family in the state of Zulia, Venezuela, have been followed prospectively for 7 years. Between 1981 and 1988, 593 people were examined, of whom 128 had symptomatic HD and 171 persons at risk had examination abnormalities that were insufficient to meet criteria for diagnosis. The remaining 294 had normal examinations. Abnormalities of saccadic eye movement and slowness of rapid alternating movements were the most common abnormalities found in at-risk individuals. Thirty persons who did not meet criteria for diagnosis at their first examination have subsequently been diagnosed with symptomatic HD. Their average age at diagnosis was 33.5 +/- 8.3 (SD) years. The likelihood of developing symptomatic HD within 3 years was 3% for those persons with normal first examinations, 23% for those with mildly abnormal first examinations, and 60% for those with highly abnormal first examinations. The rate of disease progression in early symptomatic cases were 1.4 +/- 0.1 (SEM) points per year on the Shoulson-Fahn functional capacity scale. Paternal or maternal inheritance did not appear to affect the rate of progression in this group of individuals. The data suggest that there is not a discrete age of onset but rather a prolonged period of time during which symptoms unfold.