We compared the diagnostic yields of ultrasound-guided aspiration biopsy, sputum cytology, and fiberoptic bronchoscopy with biopsy in 30 patients with peripheral pulmonary nodules smaller than 3.0 cm in diameter. Among them, there were 4 minute nodules with diameter less than 1.0 cm, 12 between 1.1 to 2.0 cm, and 14 between 2.1 to 3.0 cm. The final diagnoses in these 30 patients, as confirmed by histologic findings, microbiology, and clinical follow-up, revealed 24 malignant lesions and 6 benign. All of these 30 nodules received ultrasound-guided transthoracic fine-needle aspiration biopsy, and confirmative diagnoses were obtained in 27 (90 percent). Twenty-two (92 percent) of 24 patients with malignant nodules had positive cytology for malignancy and 5 (83 percent) of 6 patients with benign lesions had diagnosis made by cytologic and microbiologic examinations. One patient developed asymptomatic pneumothorax after needle aspiration. The size of the lesions did not affect the diagnostic yield and complication rate. None of the lesions was diagnosed by sputum cytology (0 of 19; 0 percent). Two patients had diagnoses obtained by fiberoptic bronchoscopy with biopsy (2 of 10; 20 percent). We conclude that ultrasound-guided aspiration biopsy is a useful and safe method for diagnosis of peripheral pulmonary nodules, even when the size of the nodule is less than 3 cm in diameter. The diagnostic yield far exceeds that which can be obtained by sputum cytology and fiberoptic bronchoscopy with biopsy.