Five cases of uninodular bronchioloalveolar cell (BAC) carcinoma are reported. These 5 cases were characterized by the protracted course of the disease: 14.5 and 11.5 years respectively in the 2 patients who died; 7.5, 12 and 12.5 years respectively in the 3 patients who are still alive. Prior to being diagnosed at excision, the tumours had been followed up for an unusual 2 to 10 years period, with phases of radiological stability and phases of very slow growth. This very slow course is exceptional in lung cancer. It differs not only from the course of diffuse BAC carcinoma, but also from the habitual course of uninodular BAC carcinoma and from that of peripheral adenocarcinoma. Similar cases could be found in the literature, and a comparative study has been conducted between these slow-course carcinomas and normal-course carcinomas. Whatever their outcome, slow-course carcinomas are asymptomatic tumours which present at radiography of the chest as a peripheral round or oval opacity devoid of pathognomonic features. The post-operative prognosis of slow-course tumours seems to be more favourable than that of the other lung cancers. At histology, carcinomas with a protracted course usually show a pattern similar to that of lesions called by some authors "benign adenomatosis"; however, some of these tumours exhibit fairly distinct signs of malignancy, and electron microscopic images also are far from being uniform.