This subject interests the practitioner as it will enable him to predict, at the time of diagnosis, what are the chances for the treatment being successful, how long will the survival be, etc.? Unquestionable factors are the extent of the tumour expressed as stage, the number of metastasis sites and the histological nature of the cancer. Possible factors are: 1) In non-small cell carcinomas: Stage 1: P 53 mutation, satellite nodules found at surgery, tumour size less than 2 cm in diameter. Operated cancers: absence of invaded lymph nodes, small size of the tumour, mutation on P 185 Neu, codon 12 mutation of the cancer ras, mutation of P 53. Effectiveness of chemotherapy: positive NSE labeling. 2) Small cell carcinomas: extent of the lesions, number of metastasis sites and performance index. When these prognostic factors are known, they must be taken into account for the attempted treatments. On the other hand, factors of poor prognosis may influence the therapeutic approach.