To the Editor: It is informative to contrast the results of the two clinical trials comparing bypass surgery and angioplasty (Oct. 20 issue)1,2 with the results of a previous observational study.3 Neither clinical trial found any difference in mortality rates at one year between coronary bypass surgery and angioplasty. However, the observational study found that the patients who underwent coronary bypass surgery had a higher mortality rate, even after adjustment for risk factors. The difference was greatest for low-risk patients. Differences between the studies may have been due to chance, but there are also potential structural reasons for the. © 1995, Massachusetts Medical Society. All rights reserved.