One hundred eight-five patients with potentially operable lung cancer were prospectively evaluated by computed tomographic scanning of the mediastinum and upper part of the abdomen. Mediastinal lymph node size was correlated with operative and pathologic findings. There was close agreement between computed tomographic estimate of size and operative measurements. Mediastinal lymph nodes that were larger than 2.0 cm were positive for tumor in 69.6%, nodes between 1.1 and 1.9 cm contained metastases in 31.8%, and nodes less than 1.0 cm were positive in only 2.7%. The cell type, T status, and location of the primary tumor did not influence these findings: A node less than 1.0 cm with a T3 lesion had the same probability of being abnormal as with a T1 lesion, although predictably, those patients with T3 and central tumors had a greater likelihood of having nodes larger than 2.0 cm. The presence of pneumonitis did not increase the prevalence of enlarged, histologically normal nodes. Asymptomatic adrenal metastases were present in 3.2% of patients with otherwise operable disease and were suggested only by the computed tomographic scan. Patients with mediastinal nodes less than 1.0 cm probably do not need preresection mediastinal exploration. Those with nodes larger than 2.0 cm should not be considered unresectable without pathologic confirmation, even in large tumors, in view of the 30.4% negativity rate. The computed tomographic scan is useful in depicting and localizing enlarged mediastinal nodes but cannot be used as a substitute for pathologic examination.