The frequency with which pleural adhesions are found at necropsy and accidentally by roentgenologic examination would indicate that the milder form of pleurisy is common. Undoubtedly patients often recover without the diagnosis of pleurisy having been made. Because of the obvious difficulty of classifying the milder cases of pleurisy, it is not feasible to include them in a study of the prognosis in cases of idiopathic pleurisy, although they may be of the same etiology and type as those of pleurisy with effusion. If it were possible to classify the cases accurately, conclusions regarding the outcome of primary pleurisy would be greatly modified. This study is limited to the primary or idiopathic pleurisy with effusion in which there is an appreciable accumulation of serous fluid, and does not include the idiopathic pleurisy of lesser degree.
The etiology is generally considered to be tuberculous and, although the condition is termed primary