This article describes the results of a dual diagnostic procedure, thoracentesis and pleural biopsy with a Cope's needle, in 414 patients with pleural effusion of unknown origin. A diagnosis of neoplasia or pleural tuberculosis was obtained in 241 subjects (149 with neoplasias and 92 with pleural tuberculosis). In an additional 55 patients, a diagnosis of tuberculosis or neoplasia was obtained using other procedures (15 with tuberculosis and 40 with neoplasias). In 105 subjects, the effusion was neither tuberculosis nor neoplasia. Thirteen patients were excluded from this study due to the impossibility of follow-up. The diagnostic process was repeated in 64 patients. Complications occurred in 46 patients (11%), of which 42 were pneumothorax. The dual diagnostic sensitivity in our series of thoracenteses and pleural biopsies made with a Cope's needle was 86% in tuberculosis and 79% in neoplasia with 100% specificity. The probability of a case being neither tuberculosis nor pleural neoplasia (negative predictive value) when the liquid and the pleural biopsy specimen are nonspecific (each procedure having been applied only once on each patient) is 56%, although a negative result does not exclude these diagnoses. In our opinion, the repetition of the dual procedure is indicated considering the scant morbidity and zero mortality.