Objective To explore the significance of the application of cell blocks of pleural effusion combined with immunohistochemistry in the diagnosis of lung carcinoma and detection of EGFR gene mutation. Methods After the pleural effusion specimens were centrifuged and smeared routinely, the cell sediments were embedded in paraffin to make cell blocks. Combined with immnohistochemistry technique, the definite diagnoses were made. EGFR mutations of cell blocks of 175 cases with lung adenocarcinomas were detected by ARMS-PCR method and compared with the mutations of EGFR gene of biopsy or operation spcimens of 305 cases of lung adenocarcinomas. Results The cancer cell detection rate of cell blocks combined with immunohistochemistry was higher than that of the conventional smears (P < 0.05), and the ratio of uncertainty was lower than that of the latter (P < 0.05). There was no significant difference in the positive rate of EGFR gene mutation or the male-to-female ratio between cell blocks and tumor tissues (P > 0.05). Conclusions Pleural fluid cell block can be a good substitute for solid tumor tissue, which is helpful in identifying the tumor type and source. And it can be used for the detection of EGFR gene mutations in lung carcinomas, which can effectively guide the molecular targeted therapy.