Among diffuse gliomas, oligodendrogliomas may account for 25% of cases. They have a better prognosis and chemosensitivity as compared to astrocytomas. Genetic studies have shown a correlation between oligodendrocyte phenotype and presence of 1p/19q deletions. In addition, these deletions are of prognostic value. The aim of the present study was to describe a new method to detect 1p/19q deletions when little tumoral material is available (stereotactic biopsies (SBs)). Since smears (cytological preparations) are routinely done for intraoperative diagnosis of gliomas, we have searched for 1p/19q deletions by FISH in a series of 30 patients with a glioma. In 14 cases, loss of heterozygosity (LOH) analysis was also performed in order to validate our method. We found that FISH analysis on frozen smears was a simple, rapid and reliable method to detect 1p/19q deletions and a good concordance was found with LOH data (85%). The main advantages of FISH analysis on frozen smears are the following. First, it requires little material and can be easily done in the case of SBs. Second, it has a higher sensitivity than LOH especially in infiltrative areas of gliomas. Third, it allows detection of a codeletion 1p/19q in a single tumor cell. In contrast, LOH analysis is easier to interpret and can detect smaller and partial deletion whose pronostic significance remains to be defined. In conclusion, these two techniques can be used to investigate 1p/19q status in gliomas. The appropriate choice of one or other of these two techniques will depend on the specific questions that need to be answered.