An Irish kindred is described in which 5 of 10 siblings in the fourth or fifth decade of life developed an akinetic-rigid syndrome clinically indistinguishable from idiopathic Lewy body Parkinson's disease. Four of these patients were scanned by positron emission tomography (PET) with [18F]dopa after clinical diagnosis and in all, a profound impairment of tracer uptake into the striatum was recorded. The fifth patient was initially scanned at a time when he was asymptomatic and normal by clinical examination. His scan showed impaired tracer uptake, indicating a subclinical defect in the presynaptic nigrostriatal system. Within months of his scan, he too developed subtle symptoms and signs of parkinsonism although there was little further clinical progression or change in his PET scan over the following year. A clinically normal sibling was also scanned and found to have subclinical impairment of [18F]dopa uptake in the putamen. The 19-year-old daughter of an affected sibling had a mild postural tremor but no other symptoms or signs. The [18F]dopa uptake in her putamen fell at the borderline between normal and parkinsonian values. This study confirms that PET can identify preclinical parkinsonism in at-risk subjects. The finding of abnormalities in several clinically unaffected family members suggests that family studies based on clinical assessment alone may miss a significant number of subclinically affected individuals, leading to an underestimate of any genetic component to Parkinson's disease.