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Preoperative pulmonary assessment has two goals: to help decide whether the proposed pro-

cedure is justified on the basis of its risk:benefit ratio, and to help identify features that may

be improved to reduce risk. Benefit is based on an understanding of the natural history of the

underlying disease to be treated and the likelihood that the proposed intervention will

improve that course. The accuracy of this estimate varies considerably for different con-

ditions and interventions. Risk is the observed incidence of complications resulting from

the proposed intervention. When derived from the review of large numbers of such interven-

tions, this risk can be predicted with great accuracy for average populations. However, the

considerable variability among risk subgroups limits the usefulness of such unstratified data

for the individual. Unfortunately, the enormous number of possible combinations of circum-

stances and comorbidities makes compilation of risk tables impractical for every possible

patient. Therefore, risk assessment is always a population-derived estimate to be informed

and guided by knowledge of individual patient variables and by sound judgment.