Computers are being investigated as diagnostic aids in many fields of medicine. Models employing Bayes' theorem, a statistical formula, commonly are used to supply valuable information on the likelihood of each disease in the differential diagnosis to help the clinician make the diagnosis. However, knowledge of elementary decision analysis is beneficial to help understand the current and potential uses of these models. We discussed Bayes' theorem as an introduction to decision analysis. Moreover, we described a Bayesian model for the differential diagnosis of leukocoria to illustrate the application of computers to ophthalmologic diagnosis.