The evolution of pleural effusion after nondiagnostic thoracoscopy (THS) at long-term follow up is not well known. The aim of this study is to assess the true sensitivity at long-term follow up of nonspecific pleuritis diagnosed during THS under local anesthesia in an unselected population of patients with unexplained exudative pleural effusions. A retrospective study with long-term follow up (minimal 24 months) was performed of 709 patients who underwent THS between 1982 and 1999 for an unexplained exudative pleural effusion after (repeated) thoracocentesis. Of 709 patients, 391 (55%) had a nonmalignant pleuritis. Of these, 183 (26% of the total group) had pleuritis resulting from a true benign disease. In 208 patients (29% of the total group), the cause of pleuritis remained inconclusive. After long-term follow up, the cause of pleural effusion in this last group appeared in 31 patients (4.3% of the total group). After long-term follow up, the sensitivity of diagnostic THS was 91% and the specificity was 100%. The positive predictive value was 100% and the negative predictive value was 92%. In 15% of patients, the diagnosis of nonspecific pleuritis after THS appeared to be false-negative after long-term follow up.