Procalcitonin (PCT), a propeptide of the hormone calcitonin, is a novel marker of the inflammatory response to infection. It has been used to discriminate between infectious and non-infectious causes of inflammation, and as a marker of severe sepsis in the intensive care unit.To evaluate the utility of PCT in distinguishing community-acquired pneumonia (CAP) due to common bacteria, Mycobacterium tuberculosis and Pneumocystis jirovecii in a high human immunodeficiency virus (HIV) prevalence setting.Two hundred and sixty-six patients admitted with a diagnosis of CAP were investigated. Serum samples for PCT were collected on admission. PCT levels were measured using a commercial immunoluminometric assay.A microbiological diagnosis was obtained in 169/266 patients: 44 pulmonary tuberculosis (PTB), 31 P. jirovecii pneumonia (PJP), and 35 bacterial pneumonia. The PCT levels were PTB 4.16 ng/ml (SEM 1.197; 95% CI 1.749-6.579); PJP 1.138 ng/ml (SEM 0.2911; 95% CI 0.543-1.734); and bacterial pneumonia 19.48 ng/ml (SEM 5.64; 95% CI 8.021-30.938, P < 0.0004). Thirty-six had co-infections.PCT levels differ significantly in patients with CAP due to TB, PJP and bacteria. PCT may be important in distinguishing M. tuberculosis and PJP in a high HIV prevalence setting where atypical presentations often confound the empirical clinical diagnosis.