Background and objectives: The knowledge of time-related changes occurring in cancer incidence and mortality is an es- sential element for cancer control. This paper aims to describe the time trends of the more important cancer sites in Nava- rra and Zaragoza. Methods: The influence of age, diagnosis period and birth cohort on the observed time trend cancer incidence in Nava- rre and Zaragoza was assessed using a log-linear model. Re- sults are showed graphically, for the different tumour sites by sex and each registry. Results: Among males, a considerable rise in the incidence of lung cancer (5% per year), prostate cancer (> 2% per year) and non-Hodgkin's lymphomas was found. Among females, the highest increases corresponded to non-Hodgkin's lymp- homas, with a yearly rise greater than 7%, and ovarian can- cer (4% per year). Breast cancer in women increased in both registries (3.5% per year in Navarre and 0.9% per year in Za- ragoza), part of the increment in Navarra being explained by a higher case detection rate. Finally, the incidence of colorectal, bladder and kidney cancer rose more than a 3% per year in both registries and sexes. For most types of tumours, cancer risk increased with subsequent generations. Conclusion: The substantial cancer increment observed points out the inefficacy of primary prevention policies, the importance of studying cancer incidence for long time periods and the need to increase the population coverage of Spanish cancer re- gistries.