Volume-doubling time was used as an index of growth rate for 18 solitary nodules due to bronchogenic carcinoma. This series from the Philadelphia Pulmonary Neoplasm Research Project was limited to cases whose tumors could be measured on serial chest x-ray films and in whom death, if it occurred, was attributable to the cancer. The distribution of doubling times suggests that, if lung cancer is to be detected soon after it reaches roentgenographic visibility, the interval between serial films must be less than six months in many cases. Whether such a program would improve the cure rate remains to be determined. Doubling time was plotted against survival measured from the time when the cancer was 1 cm in diameter. The slower the growth rate, the longer was the survival.