Schwannomas of the vestibular nerve are the striking characteristics of neurofibromatosis type 2 (NF2), an autosomal dominant hereditary disease. The NF2 gene on 22q12 has been isolated as the gene responsible for NF2. Previous studies have reported that 60% of sporadic schwannomas showed inactivation of the NF2 gene, but genetic alterations of remaining 40% tumors remain elusive. Moreover, detailed genetic alterations of this tumor remain an open question. In this study, we analyzed genomic alterations in 17 sporadic schwannomas using comparative genomic hybridization (CGH). Loss of chromosome 22q, including the NF2 locus, was the only notable abnormality (5/17, 29%). Further, we performed fluorescence in situ hybridization analysis with a genomic BAC clone harboring the NF2 gene and found that the 5 tumors with loss detected by CGH as well as three cases without such a detectable loss by CGH, or a total, 8/17 (47%), showed loss of the NF2 locus. Mutation search by PCR-SSCP followed by direct sequencing revealed that 71% (12/17) of the tumors had one or two mutations in the NF2 gene. Our analyses disclosed that 14 (82%) of 17 tumors had structural alteration of NF2; among these 14 cases, 9 (64%) had two inactivating mutations in the NF2 gene, either a somatic mutation in one allele coupled with loss of the other allele or two independent somatic mutations. Our present results suggested that (i) most of the sporadic schwannomas have two-hit mutations in the NF2 gene, and (ii) NF2 is the only major causative gene in the genesis of schwannomas that is activated or inactivated by copy number alterations.