Sixty-five solitary cavities of the lung were evaluated for wall thickness. All lesions in which the thickest part of the cavity wall was 1 mm were benign. Of the lesions whose thickest measurement was 4 mm or less, 92% were benign. Of cavities that were 5-15 mm in their thickest part, 51% were benign and 49% malignant. Of those over 15 mm thick, 95% were malignant. Measurement of the thickest part of te cavity wall gives a more reliable indication of benignancy or malignancy than measurement of the thinnest part.