α-Internexin (INA) has been proposed as a biomarker of oligodendroglial tumors with the 1p/19q co-deletion. On the other hand, sequence studies have recently linked the CIC mutation and subsequent altered CIC expression to the 1p/19q co-deletion in oligodendroglial tumors. We assessed the usability of combination immunohistochemical analysis using CIC and INA as a surrogate tool for the 1p/19q status in 39 cases of oligodendroglial tumors. The positive expression of INA was observed in 10 cases (52 %) of oligodendroglial tumors with the 1p/19q co-deletion, and in only 3 cases of oligodendroglial tumors without the 1p/19q co-deletion (15 %, P = 0.012). The lack of CIC expression was detected in 13 cases (68 %) of oligodendroglial tumors with the 1p/19q co-deletion, and in only 1 case of oligodendroglial tumors without the 1p/19q co-deletion (5 %, P < 0.0001). Combined immunohistochemical analysis assessed by INA expression and/or the lack of CIC expression was strongly associated with the 1p/19q co-deletion in oligodendroglial tumors, indicating a potential surrogate marker of the 1p/19q state. Although combined immunohistochemical analysis cannot be totally replaced by molecular genetic analysis as a definitive diagnostic technique, it may contribute to a steady morphological diagnosis by predicting the 1p/19q state in oligodendroglial tumors.