The phosphatidylinositol 3'-kinase pathway is activated in multiple advanced cancers, including glioblastomas, through inactivation of the PTEN tumor suppressor gene. Recently, mutations in PIK3CA, a member of the family of phosphatidylinositol 3'-kinase catalytic subunits, were identified in a significant fraction (25-30%) of colorectal cancers, gastric cancers, and glioblastomas and in a smaller fraction of breast and lung cancers. These mutations were found to cluster into two major "hot spots" located in the helical and catalytic domains. To determine whether PIK3CA is genetically altered in brain tumors, we performed a large-scale mutational analysis of the helical and catalytic domains. A total of 13 mutations of PIK3CA within these specific domains were identified in anaplastic oligodendrogliomas, anaplastic astrocytomas, glioblastoma multiforme, and medulloblastomas, whereas no mutations were identified in ependymomas or low-grade astrocytomas. These observations implicate PIK3CA as an oncogene in a wider spectrum of adult and pediatric brain tumors and suggest that PIK3CA may be a useful diagnostic marker or a therapeutic target in these cancers.