This test detects copy number changes of the short arm of chromosome 1 (1p) and the long arm of chromosome 19 (19q) using fluorescence in situ hybridization (FISH) in formalin-fixed paraffin-embedded tissue specimens. FISH is performed using two separate hybridizations, each with two FISH probes. One hybridization employs a probe specific to 1p36 along with a control 1q25 probe. The other employs a probe specific to 19q13 with a control 19p13 probe. The presence or absence of a deletion is determined by the signal ratio of each respective probe pair. 1p/19q co-deletion occurs in 44-69% of classic oligodendrogliomas and 21-33% of mixed oligoastrocytomas. This alteration is relatively infrequent in pure astrocytomas (6-8%) and can therefore be useful in the classification of gliomas. In addition, 1p/19q co-deletion is associated with more favorable prognosis and response to specific chemotherapeutic and radiotherapeutic regimens - particularly in anaplastic oligodendroglioma or oligoastrocytoma.