Ultrasound-guided fine-needle aspiration biopsy (US-guided FNAB) was performed in 40 patients with solitary pulmonary nodules (SPNs) for evaluation of diagnostic results and complication rates. The final diagnoses of the 40 patients included 30 malignancies and 10 benign lesions. Using US-guided FNAB, the diagnostic yields were 97% (29/30) in malignancies and 60% (6/10) in benign lesions. Of the 29 patients with cytologically proven malignancies, 12 underwent surgical resection. The correlation between cytological results and histologic diagnoses in these 12 was excellent (100%). The size of the nodule did not affect the diagnostic rate or complication rate. Only two patients (5%) developed minimal pneumothorax after US-guided FNAB. We conclude that US-guided FNAB is a useful, safe, and convenient diagnostic tool for SPN, and that malignant pulmonary nodules are more easily diagnosed than benign nodules.