The aim of our study was to evaluate the prognostic value of serum procalcitonine (PCT) assay in adult respiratory infections. Forty-nine patients admitted with pleurisy, community-acquired pneumonia, tuberculosis, infection were included in this prospective study. PCT was assayed on admission and discharge. Biological and clinical parameters of gravity were also evaluated. Twenty patients had elevated PCT of more than 0.50 ng/ml. In 29 patients, PCT was undetectable. The serum PCT level was normal in the patients with tuberculosis, infection, pneumocytosis. PCT did not correlate with the biological and clinical markers of the disease severity but the evolution of PCT correlated with the evolution of C-reactive-protein (r = 0.58, p < 0.05). PCT seems to be an early marker of the evolution of respiratory infections, but it does not help to establish prognosis. Further studies are necessary to assess the potential value of PCT in more severe respiratory infections requiring assisted ventilation.