['To assess the ability of endobronchial ultrasonography (EBUS) using a guide sheath (EBUS-GS) to diagnose peripheral pulmonary lesions.', 'We devised a technique for EBUS-GS covering a miniature probe, and 150 lesions were evaluated in a prospective open study. In this procedure, the probe covered by a guide sheath is introduced into the lesion via the working channel of a bronchoscope. The probe is withdrawn, while the guide sheath is left in situ. A brush or biopsy forceps is introduced through the guide sheath into the lesion.', 'One hundred sixteen of 150 EBUS-GS procedures (77%) were diagnostic. Cases in which the probe was located within the lesion had a significantly higher diagnostic yield (105 of 121 cases, 87%) than when the probe was located adjacent to it (8 of 19 cases, 42%) [p < 0.0001, chi(2)]. The diagnostic yield from EBUS-GS in lesions </= 10 mm (16 of 21 lesions, 76%), >10 to </= 15 mm (19 of 25 lesions, 76%; p = 0.99, chi(2)), >15 to </= 20 mm (23 of 35 lesions, 66%; p = 0.41, chi(2)), and > 20 to </= 30 mm (33 of 43 lesions, 77%; p = 0.96, chi(2)) were similar, demonstrating the efficacy of EBUS-GS even in lesions </= 10 mm in diameter. In 54 of 81 lesions </= 20 mm, fluoroscopy was not able to confirm whether the forceps reached the lesion. However, the yield was the same with (67%, 18 of 27 lesions) and without (74%, 40 of 54 lesions) successful fluoroscopy (p = 0.96, chi(2)). Moderate bleeding occurred in two patients (1%); there were no other complications.'] Does endobronchial ultrasonography using a guide sheath increase the ability to diagnose peripheral pulmonary lesions endoscopically?