Inactivation of p16 has been reported as one of the more frequent events in human carcinogenesis. In order to contribute to the knowledge of the impact of p16 silencing by promoter methylation, we have investigated p16 expression and inactivation of p16 by methylation in two of the major types of human cancer, non-small cell lung cancer (NSCLC) and colorectal cancer (CRC). p16 expression was evaluated by Western blot, and p16 promoter methylation by a methylation-specific PCR procedure (MSP). Clinical correlations were established using the chi-square test, and distributions of disease-free survival (DFS) were estimated with the Kaplan-Meier method. Analyses for p16 revealed that 61.22% (60 of 98) of NSCLCs, and 32.9% (26 of 79) of CRCs here considered, lacked p16 expression. Moreover, 36.7% (22/60) of the non-small cell lung tumours without p16 expression showed p16 promoter methylation, detecting a significant correlation between p16 methylation and the histological subtype of squamous cell carcinomas (SCC) (P=0.04). With respect to CRCs, p16 promoter methylation was observed in 26.9% of tumours that lacked p16 expression (7/26), all tumours studied showing partial methylation. Survival studies demonstrated a clear correlation between p16 negative expression and poor prognosis in NSCLC patients. Moreover, we found a trend toward poor clinical evolution in the group of patients with tumours showing total p16 methylation, in NSCLC, without statistically significant differences in CRC. In conclusion, our results indicate that p16 alterations constitute a major molecular abnormality in NSCLC with a considerable prognosis impact, promoter methylation being an important mechanism involved in p16 silencing. In CRC, however, p16 promoter methylation could be considered as a less definitive molecular factor without prognostic implication, since partial methylation constitutes a prevalent mechanism.