Introduction: The system of knowledge on which the practice of oncology is based was strengthened when randomized clinical trials (RCTs) replaced institution-based observational studies as our method for testing hypotheses about the effectiveness of treatment, but, there remains a void between the world of knowledge and the world of practice. Registry-Based Observational Studies (RBOs) can help to bridge this gap. Population-based cancer registries were originally established to study the epidemiology of cancer. RB0s exploit the existence of cancer registries to study the epidemiology of cancer treatment. Since RBOs deal with unselected cohorts of cases, they are free of the problems of referral bias and selection bias which contaminate institution-based observational studies. RBOs and RCTs are complementary, rather than competing, methodologies. RCTs are primarily designed to tell us how to manage individual patients, while RBOs are primarily designed to tell us how to manage the health care system. RBOs have the potential to tell us about

1. the spectrum of patients for whom the health care system must provide care;
2. the services which a specific group of patients actually uses,
3. the way medical practice is affected by
* new knowledge (eg, results of RCTs and meta-analyses),
* strategies to disseminate knowledge (eg, treatment guidelines),
* factors which may enhance or limit access to care (eg, outreach programs, waiting lists), and
* socioeconomic factors
and
4. the way the outcome of a given disease is affected by
* differential adoption of treatment philosophies in different regions,
* the introduction of new treatments,
* factors which enhance or limit access to care, and
* socioeconomic factors.

Objective: The usefulness of the method will be illustrated using the results of RBOs of the management of cancer in the Canadian province of Ontario.