A written examination was used to assess the knowledge base of 183 practicing certified internists. Analyses of the examination scores showed that performance on the initial American Board of Internal Medicine certification examination taken 7.6 years previously was the major factor predicting current knowledge base. By developing regression models, the unique contribution of different variables to prediction of current examination scores was determined. Prior American Board of Internal Medicine certification examination performance accounted for 70.9% of the explained variance, and demographic and practice variables were responsible for 17.8%. Among the demographic and practice variables studied, community size and subspecialty practice were the only variables that contributed significantly to the regression equations. Examination scores were highest for certified internists practicing in smaller communities. General internists received higher scores than subspecialists. Although statistically significant, the apparent adverse influence of subspecialty practice and larger community size on examination performance was modest. Further study is needed to determine if longer periods in practice might produce different relationships between variables such as these and examination performance.