A retrospective study was performed in patients diagnosed with primary lung cancer, and admitted to the Instituto Nacional de Enfermedades Respiratorias between 1984 to 1992. One thousand and nineteen patients were studied, 636 males and 383 females. We found a higher incidence in the group among 61–70 years of age in both sexes. The highest percentage of tumors were from the adenocarcinoma variety, followed by the epidermoid carcinoma, while the small-cell carcinoma presented a lower incidence. Smoking has been associated with the development of lung cancer. Many of our patients were smokers. The highest frequency was in smokers with a smoking habit of more than 30 years and a rate of more than 10 cigarettes per day. However, an important part of the patients with cancer did not have a smoking habit, which led us to believe that there are other etiological possibility factors (genetic or environmental) that could be involved. Most of the cases of cancer were staged as stage IIIb and stage IV and the patients had an ECOG of 1–2. The main symptoms were coughing, sputum, dyspnea and thoracic pain. This data shows an increasing frequency of lung cancer in Mexico city as well as other countries, it also shows that it is going to be a serious health problem in the future. We consider that in order to improve the prognosis, it is necessary to increase the educational and orientation campaigns among the adult population with or withou a smoking history.