Using recently published data, a nomogram was constructed to estimate the likelihood of asthma following methacholine challenge. Based on Bayes' theorem, the nomogram makes use of the sensitivity and specificity of methacholine challenge to calculate the post-test probability of asthma once the physician makes a determination of the pretest probability, that is, the likelihood of asthma before the test results are considered. A family of curves is presented to cover several levels of cumulative breath units at which the test could become positive, and a single curve is presented for a negative test after 224 cumulative breath units. Separate curves are presented for smokers and nonsmokers. The estimate of pretest probability is most crucial in negative tests where likelihood of asthma is considered high, and in positive tests in patients in whom asthma is considered unlikely. Although these curves will not apply precisely to a different data base, the concept of the relationship between pretest and post-test probability helps in the interpretation of the test results and stresses the importance of using all available information in making a diagnosis.