Ultrasonography is the most useful tool for detection and evaluation of thyroid nodules. In this study, we present our classification system for ultrasonographic evaluation, which has been routinely performed since 1995. Of 1244 nodules identified by ultrasonography in 900 patients, 1145 nodules demonstrating adequate specimens on fine-needle aspiration biopsy were enrolled in the study. We stratified these nodules into classes 1 to 5 with intermediate steps of 0.5 for classes 2 to 5. Nodules classified as 3.5 or greater were evaluated as malignant, those classified as 3 were evaluated as borderline, and those classified as 2.5 or lower were evaluated as benign. Of 233 nodules evaluated as malignant, 179 (76.8%) were cytologically confirmed as malignant. Furthermore, 145 of 159 nodules (91.2%) classified as 4 or greater were cytologically diagnosed as carcinoma. Of 710 nodules evaluated as benign, 683 (96.1%) were cytologically confirmed as benign. Two hundred fifty-five nodules of 210 patients were surgically resected and pathologically examined. In this series, the positive predictive value of ultrasonographic evaluation of malignancy was 97.2%. These findings suggest that our classification system is simple and useful to facilitate ultrasonographic evaluation of thyroid nodules.