Mutations in the epidermal growth factor receptor (EGFR) gene confer it with cancer driver gene functions in non-small cell lung cancer (NSCLC). Epidermal growth factor receptor -tyrosine kinase inhibitors are effective agents against NSCLC with a mutated EGFR gene. Accordingly, many guidelines recommend the use of an EGFR mutation test in NSCLC. However, not all patients are tested in most countries where tissue samples are mainly used for the test. As of 2011, most of the patients with advanced NSCLC are tested in Japan, and the use of cytological samples has significantly contributed to this success. A portion of samples used to determine a definite diagnosis of NSCLC, either tissue samples or cytological samples, is ensured to contain cancer cells, and is then investigated by an EGFR mutation test that is applicable to both tissue samples and cytological samples. Cytological samples now account for one-third of all the samples investigated. EGFR mutation is detected in cytological samples at a similar rate with tissue samples. The criterion ensuring an EGFR mutation test to have satisfactory sensitivity and specificity for use in both tissue and cytological samples is presented. Cytological samples are valuable clinical sources being collected less invasively than tissue samples, and should therefore be extensively used in EGFR mutation testing.