The combined experience of four university medical centers with fine-needle aspiration biopsy (FNAB) of the mediastinum is reviewed. This series includes 189 cases, with 100 males and 89 females, 6 months to 86 years of age. The majority (71%) of diagnoses were neoplastic with the remainder equally distributed between nondiagnostic/unsatisfactory and nonneoplastic lesions. Malignant lymphoma and thymoma were the most frequent primary malignancies. Metastatic tumors represented the majority (60%) of neoplasms identified by FNAB. In 16% of these cases, the primary tumor was not identified. The majority (67%) of metastases were from the lung and were predominantly small cell type (52%). Sixty-six ancillary tests were performed on 51 cases (27%). Histologic correlation was available in 78 of 189 (41%) cases, with a diagnostic sensitivity and specificity of 87% to 88% for the detection of neoplasm and 82% to 83% for distinguishing benign from malignant disease. The positive predictive value for the presence of neoplasm was 97% with three cytologic false positives identified. This series is the largest to date on the utility of FNAB for lesions of the mediastinum.