The purpose of this doctoral research is to investigate the treatment of coronary artery disease in groups that are underrepresented in randomized clinical trials using the SWEDEHEART registry. In Iceland in 2007, the radial access was rarely used, compared to being used in 1/3 of catheterizations in Sweden. After percutaneous coronary interventions, the complication rate was higher in Iceland than in Sweden. Women with one-vessel disease were less likely to undergo percutaneous coronary interventions compared to men. Amongst patients with three-vessel disease or left main stem disease, women were more likely to undergo percutaneous coronary interventions but less likely to undergo open heart surgery. There was no gender difference in mortality. Multi-morbid patients seventy years old or older with complex health needs and ST-elevation myocardial infarction (STEMI), had a significantly lower risk of one-year primary event (death, acute coronary syndromes, stroke or transient ischemic attack), if they received invasive treatment compared to those who were in the non-invasive group. Almost all nonagenarians that underwent catheterizations in Sweden during nine years were included in the last study, of whom most had at least one significant stenosis and the majority had multi-vessel disease. Percutaneous coronary interventions in nonagenarians almost exclusivly had acute indications and in-hospital complication rate was not far from the rate in younger patients. The results from the study of multi-morbid older people with STEMI are concordance with randomized studies in younger healthier patients. With the aging of the western population, it is important to study the effectiveness of treatment proven to benefit younger patients.