Recent reports have suggested an important clinical role for hypermethylation of the O(6)-methylguanine-DNA-methyltransferase (MGMT) promoter in patients with glioblastomas. Whether MGMT protein expression is correlated with promoter hypermethylation and patient outcomes, however, has not been elucidated. Here we describe a quantitative digital method for assessment of MGMT-specific immunostaining, and analyze the relationship between expression levels and methylation status of the MGMT promoter. We investigated 46 tumors from patients who received a diagnosis of glioblastoma or gliosarcoma. Immunohistochemistry with anti-MGMT antibody and methylation-specific PCR using bisulfite-modified tumor DNA were performed. The digital assessment method used image-analysis software to determine a digital MGMT staining index, and the results were compared with those obtained via conventional visual assessments. The digital staining index clearly correlated with the methylation status of MGMT promoter. In addition, the index correlated with our observational results when nuclear and cytoplasmic staining were assessed in three different fields. Our digital assessment method enabled us to assess uncertain immunopositive samples objectively and quantitatively, which is an important consideration when examining heterogeneous cellular staining. We expect that this method will be useful for assessment of heterogeneous staining with any antibodies.