The typical US pattern of obstructive atelectasis consists in a triangular hypoechoic area with anechoic bands inside related to fluid-filled bronchial structures--the US fluid bronchogram sign. According to some authors, this US sign within a chest mass indicates pulmonary parenchyma disease. Furthermore, it suggests the diagnosis of lung collapse. Sixty-one patients with obstructive atelectasis confirmed with conventional radiography, conventional and computed tomography, and bronchoscopy were submitted to B-mode and color-Doppler US to assess the importance of the US fluid bronchogram sign in obstructive pulmonary atelectasis. In this condition, B-mode US showed tubular anechoic bands in 59/61 patients. Power Doppler venous sampling showed a Doppler spectrum with marked phase oscillations. Arterial sampling showed a Doppler spectrum with high distal impedance-with poor or totally absent diastolic component. To conclude, in the atelectasis area, B-mode US showed in 96% of patients some anechoic bands with no apparent pulsatility. Color-Doppler showed color flow in 100% of cases, which confirmed the vascular nature of the masses. Thus, the US fluid bronchogram, which is frequently described in the literature, was never observed in our series. Power Doppler spectral flow analysis can be useful in the diagnosis of obstructive atelectasis because it depicts the hemodynamics of atelectasis parenchyma. Indeed, the arterial spectrum with high distal resistance is consistent with the effects of hypoxia on intra-atelectatic blood vessels. Further research is necessary to assess the role of color-Doppler US in the hemodynamic study of intra-atelectatic vessels. However, our preliminary results open new perspectives for the acquisition of physiopathologic data on abnormal blood flow in obstructive atelectasis.