Relatively little attention has been directed to the provision of health care services when demand exceeds availability. Since "waiting lists" are characteristic of the delivery of cardiovascular services in Manitoba, we hypothesized that the highest priority would be given to cases with the greatest urgency. This study examined the waiting lists for cardiac catheterization in one of two tertiary health care facilities offering comprehensive cardiovascular care to a population of slightly more than one million persons. Hospital records of all patients undergoing cardiac catheterization from May 1981 through December 1982 were abstracted retrospectively. For 871 patients entering a catheterization laboratory by two different routes (Elective Care, N = 557; Immediate Care, N = 314), patient need for immediate catheterization was assessed. Clinical differences between patients in the two groups were striking. Immediate Care patients more frequently had acute congestive heart failure, prior aortic valve surgery, and chronic obstructive pulmonary disease. Immediate Care patients were most frequently in Class 4 of the NYHA functional classification and were more often treated with triple medical therapy. These clinically ill individuals were more likely to enter the hospital via the Emergency Room; they were more likely to have long hospital stays and to die in hospital. As implemented in one Manitoba hospital, the waiting list process appears to have worked fairly well; cardiac arrest, acute myocardial infarction, and death among patients waiting for catheterization were all rare events. Both those patients needing immediate care and those who could wait with a low probability of a poor outcome were successfully identified.(ABSTRACT TRUNCATED AT 250 WORDS)