Radiographs generated during 20 years of biennial chest radiography of 107 patients with histologically proven lung cancers were reviewed for radiological evidence of slow-growing lesions. Twenty-nine of 37 solitary circumscribed peripheral masses which doubled in volume in five or more months prior to any therapy were considered slow growing. By these criteria, 7 (17%) of 41 squamous cell carcinomas were slow growing; whereas 22 (42%) of 52 adenocarcinomas grew slowly. There were no slow-growing tumors among the 14 anaplastic carcinomas or cancers of other histological type. The mean doubling time for the seven squamous cell carcinomas was 5.7 months; that for the 22 adenocarcinomas was 13.1 months. The cancers of females tended to grow more slowly than did those of males. There was no appreciable difference between the mean age of the 29 patients with slow-growing lesions and that of the remaining 78. Not only was the majority of cancer in the present study readily detectable by radiographic screening of the chest; patient survival correlated very well with the growth rates of the lesions, including those of seven resected tumors. The survivals of the seven patients with adenocarcinomas whose lesions were resected were no better than those of the remaining adenocarcinoma patients who had no surgical treatment. The results of this study proved that an appreciable percentage of lung cancers do develop slowly, especially adenocarcinomas.