The code 163.09 indicates a primary malignant pleural neoplasma-malignant mesothelioma. We have reinvestigated 173 patients, who were discharged from a department of pulmonary medicine with this code number during a 8 1/2 year period. A revision during which repeated biopsies, course of the disease and, in some cases, autopsy were considered, revealed that 63 were confirmed as having a malignant mesothelioma, 94 had neoplastic disease secondary to malignancy elsewhere, 13 had benign changes and in three cases the records were missing. Malignant mesothelioma entitles the patient to compensation if he has worked with asbestos. An occupational history was present in 94% of those suffering from mesothelioma and in 71% of the remaining patients. Among patients with mesothelioma a history of asbestos exposure was obtained in 44%, a history of no exposure in 22% and no specific mention of asbestos in 34%. In the group of patients who did not have mesothelioma 14% had known exposure to asbestos, eight none and in 78% no specific information concerning asbestos was available. Twenty-three of the 63 patients with mesothelioma had been notified to the workmens' compensation board. Retrospectively, we found that 12 more patients should have been notified. Eight patients who did not fulfil the criteria for malignant mesothelioma had erroneously been notified to the board for compensation. We find that code 163.09 frequently has been used as a working diagnosis, which could not invariably be substantiated. Although malignant mesothelioma had been suspected, previous history concerning asbestos exposure was often incomplete or absent. A correct occupational history and aggressive bioptical procedures are essential in all cases where malignant mesothelioma is suspected.(ABSTRACT TRUNCATED AT 250 WORDS)