This study was conducted to investigate the value of a written knowledge test of communication skills for predicting scores on a performance test of communication skills. A paper-and-pencil test of knowledge about communication skills and a performance test of communication skills, consisting of four stations with standardised patients, were administered to students of two classes of the medical schools of Maastricht and Leiden, the Netherlands. The results on these tests were compared. From the results of both instruments, the classes of the participating students could be recognised equally well: 60% correct qualifications of the classes by the knowledge test and 64% by the multiple station examination. Between the two tests an overall, disattenuated correlation of 0.60 was found (N=133, P < 0.01), suggesting moderate predictive value of the knowledge test for the performance test of communication skills. The correlation is stronger for students from Maastricht medical school than for their colleagues in Leiden. Correlation between the knowledge of communication skills test and other available test results of the participating Maastricht students is close to zero, suggesting that the test measures a distinct quality of students' competence. The paper-and-pencil test of knowledge of communication skills has predictive value for the performance of these skills, but this value seems to be less pronounced than similar findings for clinical procedural skills. The stronger relationship between 'knowing how' and 'showing' in the Maastricht student group might be indicative of an effect of the training format.