Exonic deletions of the Parkin gene are common in the autosomal recessive form of juvenile parkinsonism. Here we report Parkin gene mutations among apparently sporadic Parkinson's disease (PD) patients. We screened 200 patients with PD (103 women and 97 men). The age of onset was 54.2+/-10.3years (mean+/-S.D.).Four out of the 200 patients had homozygous exonic deletions in the Parkin gene. The clinical features of these four patients were essentially the same as those of idiopathic PD. The age of onset was consistently younger (33, 38, 47 and 48years, respectively). On medication, all of them were at Hoehn and Yahr stage II or III even after 12-16years from the onset of the disease.Thus 2% of apparently sporadic PD patients in Japan have homozygous Parkin gene mutations. This positive rate was 6.3% among the patients with the age of onset below 50. Our study suggests that the prevalence of the carrier state of Parkin gene may be more than that we expected. Our study warrants further studies on Parkin gene mutations in apparently sporadic PD patients.