Heart size is an important and useful diagnostic parameter in chest radiographs. However, there is a large variation in the subjective judgment of cardiac enlargement (cardiomegaly). To reduce this subjective element, the authors are developing an automated system for quantitative analysis of heart size in digital chest radiographs. Four hundred chest radiographs were reviewed initially by two radiologists and were classified into two groups: those with and those without cardiomegaly. Another radiologist reviewed 47 images which were not classified consistently in the initial review. The authors used these radiographs to construct a database for determination of cardiomegaly. Numerous parameters related to heart size were obtained in a semiautomated analysis of these radiographs, and the use of each parameter for detection of cardiomegaly was evaluated by means of receiver operating characteristic analysis. The authors also examined whether the accuracy would be improved when they applied multivariate analysis to a pair of parameters. From the analyses of the individual parameters, the automatically determined cardiothoracic ratio was found to be the single most effective measure for detecting cardiomegaly in chest radiographs. However, multivariate analysis provided results superior to those with an individual parameter.