This article explores the impact of the tonsillectomy operation using a large data base and several different approaches. The likely range of effects of the surgery and various threats to validity are studied from a quasi-experimental perspective. Information on possible patient self-selection for tonsil surgery is provided. Selection is dealt with from another viewpoint by comparing the outcomes of patients under the care of physicians having a "high propensity to operate" with those going to physicians who tend to operate less. Finally, a computer simulation studies the extent to which the measured results of tonsil surgery might be explained by particular selection mechanisms working in conjunction with random processes of episode generation. The "savings" in respiratory episodes estimated by the various approaches ranges from 0.1 episodes to approximately 0.8 episode for the year following surgery.