The aim of this study was to investigate the clinical applicability of transthoracic ultrasound (TUS) in the diagnosis and follow-up of community acquired pneumonia (CAP).We designed a pilot study in 15 patients and subsequently investigated 342 patients (206 men and 136 women) consecutively admitted to our Department from September 2005 to November 2009 because of radiographically diagnosed CAP. All patients underwent standard chest radiography, and consequently TUS. Follow-up TUS were performed at 4th and 8-10th day, in most patients.Concerning the reproducibility of TUS method, no reader's bias was present (P=0·18), overall variability and between-subject variability (inter-reader agreement) did not show any difference between readers (P = 0·62 and P = 0·32 respectively), and estimated within-subject variabilities (intra-reader agreement) suggested a very high repeatability of the method (P ∼ 1). Of 342 patients with Rx diagnosis of CAP, in 314 patients (92% of cases) a pulmonary consolidation was also detected using TUS, whose ultrasonographic patterns were studied. Pleural effusion was detected in 120/342 (35%) patients using ultrasound and in 111/342 (32%) patients using chest radiography. Overall dimensional changes of the lung consolidated areas assessed with TUS method showed highly significant results. (1st day mean ± SD: 66·34 ± 19·25; 4th day: 39·92 ± 14·61; 8-10th day: 7·41 ± 1·50; P < 0·0001).TUS is easily reproducible and we proved it to be a useful complementary diagnostic tool for the diagnosis and the follow-up of CAP.