The stage of a patient's cancer at diagnosis is essential to predict the prognosis and plan the treatment. Since 2008, stage data have been collected on all Ontario patients with breast, colorectal, lung and prostate cancers and are linked to other data collected by Cancer Care Ontario. Here, an analysis of such data is presented. How it can be used to assess the value of screening programs, inform resource allocation, evaluate compliance with treatment guidelines, compare survival trends and enhance the spectrum of cancer control activities across the province is demonstrated. International comparisons can also be made.