We assessed the sensitivity, specificity, and positive and negative predictive value of methylene blue staining in the diagnosis of oral cancer in 58 patients. The sensitivity was 90%, the specificity 69%, positive predictive value 74%, and negative predictive value 87%. Because of the number of false negatives and false positives we recommend that the diagnosis should always be confirmed by histopathological examination of a biopsy specimen. Methylene blue staining may, however, be useful as a screening tool for oral cancer in large, high-risk groups in a similar way to the more expensive toluidine blue.