Abstract Glioblastoma multiforme is the most common primary malignant brain tumor and despite treatment with surgery, radiation, and chemotherapy, the median survival of patients with glioblastoma multiforme is ∼1 year. Glioblastoma multiforme explants and cell lines have been reported to overexpress the interleukin-13 receptor α2 subunit (IL13Rα2) relative to nonneoplastic brain. Based on this finding, a recombinant cytotoxin composed of IL13 ligand and a truncated form of Pseudomonas aeruginosa exotoxin A (IL13-PE38QQR) was developed for the targeted treatment of glioblastoma multiforme tumors. In a recently completed phase III clinical trial, however, IL13-PE38QQR was found to be no more effective than an existing therapy in prolonging survival. To determine possible explanations for this result, we analyzed the relative expression levels of IL13Rα2 in glioblastoma multiforme and nonneoplastic brain specimens using publicly available oligonucleotide microarray databases, quantitative real-time reverse transcription PCR, and immunohistochemical staining. Increased expression of the IL13Rα2 gene relative to nonneoplastic brain was observed in 36 of 81 (44%) and 8 of 17 (47%) tumor specimens by microarray and quantitative real-time reverse transcription PCR analyses, respectively. Immunohistochemical staining of tumor specimens showed highly variable expression of IL13Rα2 protein both within and across specimens. These data indicate that prescreening of subjects may be of benefit in future trials of IL13Rα2 targeting therapies. [Cancer Res 2007;67(17):7983–6]