Deletion of the short arm of chromosome 1 (1p) is considered a favorable prognostic factor in glial tumors. High-density array-comparative genomic hybridization analysis of 108 gliomas shows two distinct types of 1p deletions. Complete hemizygous losses of 1p, which are tightly associated with 19q loss and oligodendroglial phenotype, and partial 1p deletions mainly observed in astrocytic tumors and not associated with 19q loss. Whereas the first type predicts longer overall and progression-free survival (p < 0.0001), the second type has a pejorative prognostic value. Complete 1p-arm evaluation therefore is required to appreciate the real clinical significance of 1p loss in gliomas.