BACKGROUND Malignant pleural mesothelioma (MPM) is marked by its difficult diagnosis and poor prognosis. Medical thoracoscopy (MT) is an effective and safe procedure for the diagnosis of exudative pleural effusions and many factors associated with poor prognosis of MPM. We conducted this study to investigate the value of MT for diagnosing of MPM and to identify prognostic factors for MPM patients. METHODS From July 2005 through June 2014, a total of 833 patients with undiagnosed pleural effusions underwent MT and pleural biopsies were taken. Clinical data of all patients with MPM were retrospectively analyzed, and those with complete follow-up data were analyzed for prognostic factors. RESULTS Eventually, MPM was the final diagnosis in 40 patients. Diagnostic efficiency of MT for MPM was 87.5%, since diagnosis of MPM failed to be established in 5 patients during the initial MT. Median survival was 17.1 mo (95% confidence interval: 13.6-20.7 mo). MT findings of pleural adhesion and plaques were adverse prognostic factors for MPM. In addition, old age, male gender, smoking history, histological type, poor staging, no treatment, low total protein level in pleural fluid, and computed tomographic findings such as pulmonary consolidation or infiltration, mediastinal lymphopathy, pulmonary mass or nodules, and pleural nodularity were also poor prognostic factors for MPM. CONCLUSIONS MT is safe with a high positive rate in the diagnosis of MPM, and pleural adhesion and plaques seen under MT may be the adverse prognostic factors for MPM. Multiple clinical characteristics can affect the survival of MPM patients.