In a randomized design we examined the interobserver variation in the histopathological diagnosis of adenocarcinoma of the lung and malignant mesothelioma. In three rounds, three pathologists assessed slides from 42 tumours originally diagnosed as adenocarcinomas, malignant mesotheliomas or benign lesions in the pleura. In the first round the assessments were made on haematoxylin and eosin (H & E) stained sections; in the second, on H & E sections plus sections stained with histochemical mucin stains; and in the final round, the diagnoses were made on H & E sections and sections stained with a panel of antibodies against various antigens (cytokeratin, EMA, CEA, Ber-EP4, B72.3, Leu-M1, vimentin and S-100 protein) said to be of value in the differential diagnosis. The overall interobserver agreements for the three rounds were 0.659, 0.802 and 0.817; the kappa values were 0.461, 0.681 and 0.690. It is concluded that differentiation between adenocarcinoma of the lung and malignant mesothelioma should be made on sections stained with H & E and mucin and/or immunohistochemical staining reactions, including antibodies against B72.3, Ber-EP4 and CEA.