One hundred patients with solitary intrapulmonary round lesions were examined by CT and conventional tomography in order to compare their value in reaching a diagnosis. Correct diagnosis as to whether the lesion was malignant was possible in 79% by tomography and in 84% by CT; the type of lesion was diagnosed correctly in 50% and 64% respectively. CT was superior to conventional radiography, particularly for the recognition of calcified tuberculomas. A density value was determined which, when exceeded, always indicated that the lesion was benign.