To identify a better diagnostic criteria for oligodendroglial tumors, we investigated the clinical, histological, and immunohistochemical features that would be able to predict a 1p/19q loss of heterozygosity (LOH) in these tumors. We performed a PCR-based LOH test with the 1p and 19q microsatellite markers by microdissecting tumor in 56 samples (44 oligodendrogliomas and 12 mixed oligoastrocytomas) of paraffin-embedded tissue. Patients with oligodendroglial tumors with 1p/19q LOH had a statistically significant better prognosis for overall survival. Comparative analysis of several features indicated that the 1p/19q LOH tumors were associated with two histological features, "tumor cellularity" and "perinuclear halo," and low O(6)-methylguanine-DNA-methyltransferase (MGMT) expression and high cytoplasmic glutathione S-transferase pi (GST-pi) expression. In addition, the incidence of 1p/19q LOH was infrequent in the youngest age category (less than 20 years old) studied. Using the new features, we could predict the 1p/19q status of oligodendroglial tumors with greater than 90% accuracy. Therefore, applying these features in clinical practice, would be helpful in clarifying oligodendroglial tumor diagnosis.