Decisions about managing solitary pulmonary nodules often involve estimates of the likelihood that the nodule is malignant. We used Bayes' theorem to devise a simple scheme for estimating the likelihood that a solitary pulmonary nodule is malignant based on the diameter of the nodule, the patient's age and history of cigarette smoking, and data on the overall prevalence of malignancy in solitary nodules. This method may improve the accuracy of estimating the likelihood of malignancy for individual patients with solitary pulmonary nodules.